recruit_pool "NE Ballista" 1 0.25 1 0 requires factions { Wales, } and region_religion welsh 20

As far as i know the 0.25 here means the portion of the unit produce each turn, ie 0.25 means 4 turns to produce 1 unit.

then the 1 following is the maximum pool this unit.

I dont know what is the first 1 and last 0 stand for to be honest.