PA - PSA

Hi, HR gurus,

As am new to SAP HR, could any one help me in these scenario : Actually my client has 3 different office/plant. in that it has 1 Head office and 3 different Production plants at different area. and all these have different departments like accounts, marketing, sales, production, commercial etc..

Now could any one help to whom i should define as PA (personnel area) and PSA (personnel sub area)....???

help me plz,,

-jagdish