Call SAP function from BSP

Hi BSP Gurus,

I am new to BSP World.

I was told that review the existing BSP module and modify the SAP function module to take care of the new business unit.

Now my question is where to start ?....do I need to know the existing BSP module ? How would I know what SAP function is being called from BSP ?

Please help.

With regards,

Pranav