Field help (FLGAMT) 

Check this box to indicate that the amounts of the generated ledger must be taken from an existing ledger specified in the Ledger type field.
The following characteristics of the recovery ledger must be the same as the one of the new added ledger:

If the setup of the account core model is not relevant (i.e. does not include at least one ledger with these characteristics), the fields will not be available.

The terms for recovering amounts depending on the various setup types are detailed in the function documentation Generate entries.