Acol is a bidding system that utilises limit bids extensively and raising partner uses them.
A simple raise shows 6-9 points and four-card support. Rarely responder will have three-card support with a shortage somewhere, hoping that ruffs with the short trump suit will compensate.
A jump raise shows 10-11 points and at least four-card support. This bid is invitational.
Traditionally a double jump raise showed the values for game, say 12-16 points, but this has fallen out of favour. The double jump raise is now a pre-emptive bid, showing a long suit but few points and few defensive values. For example, over a one heart opening bid,
S x
H AQJxx
D xxxx
C xxx
I would respond with 4H. Long hearts, no defence, and trying to make life difficult for the opponents.
This raises the question of what to do with a game-forcing hand with support and, in Acol, there is no good solution. For interest other bidding systems tend to use 2NT to show such a hand, but this shows a balanced 11-12 points in Acol.
The best you can do is bid another suit (forcing) and then support partner. It is not a strong area of Acol.
In general, if there is a choice between bidding a new suit or supporting partner with a limit bid, it is right to support partner: this tells partner immediately about the support and limits your hand.