Project Invoice(PSAProjInvoice and Project Manage Invoice (PSAManageInvoice) Work Logic

Hi all, I have been customising the layout on the Default Project Invoice Report . But when Customers are adding Attachments , comments
to Lines and more . The PSAManageInvoice kicks in. Studying the Elements of both SSRS Reports   the PSAManageInvoice contains almost nothing
default ? why are they so different ?  I have read the PSAManageInvoice  =  Custom Project Invoice .  Can some one explain me the Logical setup
in this scenario.