DTC_ID_ALTERNATE is used by the payroll system to offer options during data entry of transactions (through IPTR) or through the generation of holidays (using the UPTG function).
Transactions generated in IPTR may have "Alternate Time Code" toggled on which will result in this alternate time code being used when the transactions are taken to the Pay Headers/Pay Lines using UPTR.
Holidays are normally generated using the time code specified on the Holiday Calendar (IDHC) screen. If the holiday calendar is set to be observed on the Holiday Date, and the day would normally be a day off for the employee (according to their schedule), this Alternate Time Code could be used to generate the time to a different time code.