Field help (DEPRAT) 

Early discount or late charge rate impacting the invoice amount.
For an early discount, the specified rate must be negative.
For a late charge, the specified rate must be positive.
SEEREFERTTOAn example is provided in the documentation of the function Early discount/late charge.
It describes the rule applied to determine the rate used according to the variance between the expected and actual payment dates corresponding to the open item.