A Support Double is made by Opener and shows exactly 3 card support for Responder's Suit.

Opener	quiet as mouse	Responder	Opener's RHO
1	PASS	1	2
X

Opener's hand might be something like:
A43 Q65 42 KQJ65    or
J43 AKQ4 42 AKQ4


Opener bids one of a suit.
Responder bids one of a different suit.
The Opponent next to bid bids yet a third suit. Suits everywhere. It's practically a textile industry in a bullish market.

Opener's X shows exactly 3 card support for Responder's spade suit. It denies four card support.
It in no way describes Opener's point range.

If RHO had bid 7, Opener might like to make a penalty double instead of a support double.
Then again, maybe showing 3 spades would have merit.
Responder could then decide whether or not to bid an unbeatable 7 or instead PASS if not holding sufficient length and strength in spades. With only 4 spades, the A, and 25 points, perhaps 7N would be Responderis correct choice, perhaps depending on the Vulnerability.

Your partnership may wish to limit your Support Doubles to RHO bids of 2.

Opener	quiet as mouse	Responder	Opener's RHO
1	PASS	1	2
X

In this auction, Opener promises 3 hearts for Responder.
The partnership is forced to at least 2N if Responder hasn't spades and quality enough to PASS the DOUBLE.
Such a Support Double then requires better than an opening hand.

Opener	quiet as mouse	Responder	Opener's RHO
1	PASS	1	2
X

In this auction, responder may choose to play in a 4-3 spade fit if holding less than 11 HCP.

Some play Support Doubles through 3.