What is the difference between schedule 40 and schedule 80PVC fittings?

Please explaing which has more strength?

Schedule 80 is thicker and rated for higher pressure.

Can use sch 80 pipe with sch 40 flange, then what I have to take into account

The fittings are the same. The ID of schedule 80 is smaller or the wall thickness is higher, depending if your glass is half full or half empty.