When you post cost per entry, G/L entries will be posted with original source document number. When you post per group Navision will post several document in one entry. You will have to insert some document number, that in a later date help you identity that document.
if you have 2 purchase invoices containing the same item
PI00001 ==> debit purchase, credit vendor 10000
PI00002===>debit purchase credit vendor 5000
system will post 2 entries
PI00001, debit inventory credit direct cost applied 10000
PI00002, debit inventory credit direct cost applied 5000
if you do the same per posting group one entry will be created, at the amount of 15000, having document number the one that you were asked to fill when running the batch job.
you dont have to enter purchase invoice number in this case, the system will take it automatically.
regarding per posting group, same accounts will be affected as in the previous example, but insetead of wrinting one g/l per document, they are grouped.