Best way to get possession order and claim rent arrears

Following a s21 I wish to apply to the courts for a possessiom order. I don't want to use the accelerated route as want to claim rent arrears but I've read some conflicting stuff. Some sites say a s8 is needed but NLA website says:

If the landlord has served a section 21 notice and wants the courts to order the tenant to repay rent arrears he should use the standard possession procedure.

What exactly does this mean? Would I still use form N5 or can I use PCOL?

Thanks