what is RCM

if a register person purchase from unregister or composition gst person...he need to pay all his output gst liability because no input credit available.. ex 100 rs purchase 150 sale 10%gst..so 150x10%=15rs he need to pay..process is in purchase you need to add tax 100x10%=10.so the purchase is now 100+10=110..now gst is 15-10=5rs...but its all come to the 15 rs as u only payed 10 rs in purchase....