Field help (RRB1COA) 

When the Allocation option "Account" is selected, the "Accounts" table can be modified.
In this case, the main ledger plan is set up is the "Plan" field.
Since the heading can be used for multiple companies with different main ledgers, several lines can be set up as long as the plan/account pair is unique.

For headings 000 - Net Amount and 100 - Gross Amount of Plan/account pair must not be set up.