Need some help with FI Substitution

Hi Guys,

We have a requirement where in a field of BSEG is to be substituted at call up point 3 - Complete Document.

>> When i create an exit Say U001 in ZRGGBS000 , how would the BSEG data appear as ? Will it be in the form of an internal table containing all lines for that particular document ?

>> I also have to read some lines of BSEG for a few conditions ? How would i go about doing this in the exit ? Can i say LOOP at bseg / READ TABLE BSEG ? or do i have to first get the required data from BSEG into an internal table seperately ?

Apprecitate your help !

Thanks !