Standard BC combines Documents Lines (Sales/Purchase Invoice/Credit Memo) to 1 G/L Entry if these lines have the same No. and Dimensions. An often made customization is to prevent this; end-users like to have 1 G/L Entry per Document Line.
This combining can be prevented with field 1000 "Additional Grouping Identifier" in table 49 "Invoice Post. Buffer". But this needs an extension and a similar customization might by used by other extensions, which might interfere with each other.
A solution is to add the field "Document Line No." to table "G/L Entry" and table "Invoice Post. Buffer" (part of the primary key).

Another advantage of having the "Document Line No." in the "G/L Entry" table is that it can save some extensions on already heavy used tables. Eg. it can be used to get the posted Document Line (eg. "Sales Invoice Line") and with that the "G/L Entry" can be linked to tables in extensions. Eg. if I want to "G/L Entry" to be linked to an 'entry' table in my exension, than I now have to add fields to several table, like the Sales Line, General Journal Line, Sales Invoice Line and G/L Entry. Also subscriber events might be needed to transfer data between the tables in the posting process.