Systemically an Acol 1 Spade opening shows at least 4 spades and 12-19 HCP. But, we know a lot more than that and and this bid is a good example of listening to what we are not hearing!
Partner did not open any of the suits below spades. So we know they either have 5 spades or 15+ points.
How do we know that? Because with 4 spades and 12-14 HCP the opening is 1NT.
Now your challenge is to disagree with me! Examples please.