FFCore uses XPIF for defining Printer Destination job ticketing. The DFEs do not support subset finishing via XPIF. As such, this is not supported in FFCore.
There are ways to get around this limitation. However, whether they make sense depends on your specific scenario. Do you have more details on what you are trying to enable?
After checking with my manager, I'm in the wrong forum. We don't have Freeflow Core, we've got Freeflow Makeready version 9. Sorry for the confusion.
Could you point me at an information source for Makeready?