A hard cap is one that can not be broken or exceded ever (stoneskin without gorget of mifflers is hard capped at 350HP).
A soft cap is one that can be exceded, but not easily.
EG: Cure II has a soft cap of 90hp, you can cure for 88-90Hp as soon as you get the spell while naked (like 40MND), adding a little MND will get you to 90. At 75, even with 150 MND build you will cure for something like 94Hp.
So you can go over 90, but not easily, The soft cap is the spells 'natural' curing amount and although you can go over this point its not effective; you are far better off stacking -enmity etc rather than trying to squeeze out 1-2 more hp.