Budgeting for Material Management

Hi Guys:

I am starting to get success in budget posting now 😊 and AVC gets triggered at the time of consumption in FI. This of my query related to material management area. I have a fund center "Procurement". Its is a superior fund center in procurement hierarchy. Local procurement, Technical procurement, Imports Procurements are subordinated to it. I have created their fund centers as well. I want to assign budget to these areas of responsibly for the year. Since purchasing is done through MM, i am confused what to give in Commitment item field in FMBB ? for entering budget. What thing will trigger system if i post material PR ?How will the whole process work.

Regards