Table lookup in sap po 7.4

Hi there,

   And recently we have got new PO 7.4 system , to be frank I dont have much idea on this new PO.

and the first scenario is the source input CDF file to ECC (Idoc) via PO.

we need to maintain some table (not sure where) i.e. during mapping runtime for a combination of source field (say 4 field values) from table we need to get combinaiton of target fields values and populate to the Idoc fields.

since we use single stack here my query is

1, where should we create the tables ? ECC or DB (DB2) or ?

2, which will be the best practise wrt performance ?

3, what will be best approach here?

pls suggest.