Attitude

If you hold AK953 and your partner leads the Q, how would you show your partner that you want that suit led again (i.e. a spade continuation) ?
Using STANDARD "attitude signalling", you can play the 9, highest of your spot cards.
If instead you held exactly 953, you could play the 3 to discourage partner's leading the suit again.

Count

Advanced players often estimate the suits' divisions among the four hands before the opening lead (often within two cards).
Those estimations can be aided/tested/refined if instead of attitude signalling, "COUNT signalling" is practiced.
When declarer first leads a suit, if you hold 983, play the 3, lowest, to show an ODD number of cards remaining in a suit.
If holding 9853, play the 9, high, to show an EVEN number of cards in the suit.
HIGH=even LOW=odd
You may decide to play ATTITUTE signals with beginner/INT partners and COUNT with EXPERT partners, asking beforehand to be sure.
A popular practice is to play ATTITUDE when following to your partner's first lead of a suit and COUNT when following to declarer's first lead of a suit.
Consider your partner as well as your needs in choosing a signalling system.

SUIT PREFERENCE

Yet another alternative to attitude signalling is "SUIT PREFERENCE".
When declarer leads and you hold 953 in some suit, you may agree with your partner that following with the 9, HIGH, shows preference for "the" higher ranking suit, while following with the 3, LOW, shows preference for "the" lower ranking suit.
Problem: There are 3 suits other than the one being played. One suit needs to be eliminated from consideration. If possible, rule out trumps. For example, of spades are trumps and you are 'suit preferencing' to a club lead, hearts are higher ranking than diamonds (thus the higher suit, encouraged with the 9 from 953)

Both defenders consider dummy, the auction, and cards in hand in determining which suit to eliminate.
If Clubs may be ruled out (i.e. dummy's clubs are AKQ), and Spades is the suit led, then hearts and diamonds remain to suggest a preference for (might be NoTrump contract).
To ask for a heart, the higher ranking of hearts and diamonds, discard the 9 from 953 if clubs were led and you had none.
To ask for a diamond, the lower suit, discard the 3 from 953.

ODD-EVEN FIRST DISCARD (OEFD)

Playing OEFD, a defender's first discard shows attitude or suit preference.
Suppose declarer is playing spades.
A defender, with no spades, on the first discard would discard an ODD card, perhaps the 3, to ask that a diamond be led.
ODD ENCOURAGES
An EVEN discard would tend to discourage.

If dealt two EVEN cards, discard LOW EVEN to show preference for the LOWER ranking other suit (eg. 432 to a spade lead to show preference for diamonds).
...discard HIGH EVEN to show preference for the HIGHER ranking of the other two suits (eg. 432 to a spade lead to show preference for hearts).

If discarding to a spade lead,
the 2 would tend to suggest a preference for clubs
while the 10 would tend to suggest a preference for hearts.

If a Spade were led, an even diamond discard (the other suits: hearts outranking clubs), high for hearts, 8 from 852.

If hearts were led and you had none, but held A984, you could ask for a diamond by playing the 9, odd.
You could ask for the HIGHER ranking suit Spades by discarding the HIGHER EVEN card,the 8.
You could ask for the LOWER ranking suit Clubs by discarding the LOWER EVEN 4.

What if you hold 753? Better hold on to those beauties! :) You don't really want partner leading that suit to you, do you? Odd encourages

How about AK642? You might have to keep ALL of those and maybe discard a Q from another suit to get two diamond tricks. Even discourages!

The Ten is even and the J is one higher: odd, and the Q is one higher again: even.
Maybe discard the Q from QJT98 to encourage partner's diamond lead, rather than a club.

Lavinthall DISCARD (LAV)

With LAV, ANY discard of a suit discourages the lead of that suit. A HIGH DISCARD encourages the lead of the HIGHER of the other suits.
A LOW DISCARD encourages the lead of the LOWER of the other suits.
On a heart lead holding Q932, discard the 9 to show preference for diamonds (between diamond and clubs, of course).
Discarding the 2 asks for lower ranking clubs.

Trump Suit Preference (TSP)

When following or leading trump on defence, play a low trump to encourage "the" lower suit. From the three other suits, one need be eliminated. (Sugg. Reading)

Play a higher trump to encourage "the" higher suit.
If your trumps are 42 and you and partner can rule out hearts as a suit to be considered (e.g. dummy holds AKQ), play/lead the 4 (higher of the 4 and the 2) to ask for the lead of higher ranking diamonds (of clubs and diamonds).
Play/lead the 2 in the same scenario to ask for clubs.

Trump Suit Preference Count (TSPC)

TSPC is much like TSP above, except you have to have AT LEAST 3 trump.
If your trump are 432 and hearts can be ruled out, play the 4, the higher card, to ask for the lead of the higher ranking remaining suit, diamonds.

On the next play of a spade, with 32 remaining, show count (high=even, low=odd) in another suit. Which suit though???
If you agree that you show count in clubs, play the 3 before the 2 (high then low) if you hold an EVEN number of clubs.
Playing the 2 then the 3 (low then high) would show an ODD number of clubs.

If you held 853, the 8 first would ask for the lead of "the" higher suit. The 3 instead first would ask for "the" lower suit.
If you played in this order trump 358, you would be asking for "the" lower suit and would show an ODD number of Clubs.

That's LOW, LOW, HIGH. The 3 is lower than the 5. The 5 is lower than the 8. Of the one remaining card(s), the 8 is HIGH.
It's also LOW some might argue. Neither, since there's no other card to compare it to?
At least if you call it HIGH, you can read the order backwards and it makes sense...until you get to that last card heading in the other direction.
Is this a digression?
Onwards...

What about trump in order 853?
That is HIGH HIGH LOW: The 8 is higher than the 5 and the 5 is higher than the 3. That asks for "the" higher suit to be led and shows an EVEN number of clubs.

You and your partner might decide that if club count has already been shown, count in another suit should be shown. Again, which other suit???
If clubs were trump, or you had already given count in clubs, you should signal diamond count instead.
One idea is that if club count is known, you instead show count in the next higher suit, diamonds.
If both Club and diamond suit lengths are known, then "show count" in the last non-trump suit.

What if Clubs were trump? Yikes? It's easy. If hearts are trump and diamond count is known, the lower non-trump, count unknown suit is, somehow, spades.

Why not agree (playing TSPC) one should show Lowest Non-Trump Count Unknown Suit count?
If Clubs were trump and Partner had overcalled 1 (requiring 5 diamonds), you had raised (requiring 3), and dummy had 5, you and your partner should probably show count in another suit. Obviously declarer has none and both you and partner obviously have your minimum lengths (5 and 3, respectively).

Often trumps are postponed by both sides and you and partner have shown count in a suit prior to the play of your 2nd (of 3 trumps), assume diamond count.
If Clubs are trump and diamond count known, then the Lowest Non-Trump Count Unknown Suit is hearts.
That's pretty far fetched : hearts being the LNTCUS.

Actually SPADES might be the LNTCUS!
If hearts were trump and the minor suits () lengths known, you might with a LNTCUS Spade count let partner in on spade count (and indirectly the number of trumps you hold (Total = +++). You might have a fourth trump.

LNTCUS Quiz:
643 played in order shows what?
823
293
2KJ
A32
482
QJ2
KT2
928
236
J32
523
432
234
AQJ
253

Final Caveat: Don't risk losing a trick to signal unless you lose two if you don't!