Dear Mohammed,
When you assign a subprocess to an org unit , you can choose if this control allows local maintenance “ YES “or “NOT”. If you choose "NOT", you create a local control referenced from the central catalog (P3) , which provides centralized maintenance. So , you are not able to edit the referenced this kind of controls.
If you choose "YES " , you create a local control copy from the central catalog (P2) and you can later edit the local controls.
I hope help.
Regards,
Karina Atonaidán