I have a question about lookup for the above table.
I understand that the Fieldgroup–Auto lookup is the area where we put the required fields, which System will use while looking up on the particular table.
My question is regarding the LedgerTable, I found the Field Group empty, but while creating a journal line I noticed that the lookup for Account number gives us information regarding the account number, Name of the account, search name etc…
There are many ways to build lookups like overriding the lookup method, using relations, using lookup forms…
The lookup for Account Number in journals is a bit complex because it will change the lookup based on the selected account type. For this case, lookup method of the AccountNumber field on the form datasource has been overridden. This will call the \Classes\LedgerJournalEngine\accountNumLookup method which has separate cases for different account types. Account Number in your case(when the account type is Ledger) uses LedgerAccountLookup form.