Field help (NUMDAYINV) 

If the sequence number of the document is a chronological one, and an invoice has been created with a wrong date, the error can be serious since the process cannot be reversed.
This field is used to check the entry of the document date: If the user tries to create a document (invoice) with a date later than the current date + the number of days indicated, an error message is displayed when entering the invoice.
The control is applied only n the following situations:

If the entry type has not been defined on the invoice type, the sequence number is not entered on the entry type. Therefore the automatic journal will be used to evaluate the entry type and the associated sequential number. If no entry type is defined, the entry types SIHI or BPCIN automatic journal will be checked by default.