Transaction codes.

[prataprc]

A transaction can have a textual “code”. This has no meaning and is only displayed by the print command. Checking accounts often use codes like DEP, XFER, etc., as well as check numbers. This is to give you a place to put those codes:

2012-03-10 (#100) KFC
    Expenses:Food                $20.00
    Assets:Checking

[prataprc]

Refering to ledger-cli:

By using the --payee=code option, you can generate a register report
where the payee for each posting shows the code. Alone, this is not
terribly interesting; but when combined with the --by-payee (-P)
option, you will now see account subtotals for any postings related
to a specific fund.