In my watchOS 2 app, I have support for all the complication families. For utilitarian small (CLKComplicationFamilyUtilitarianSmall) I was using the template CLKComplicationTemplateUtilitarianSmallFlat.
watchOS 3 now includes the family CLKComplicationFamilyUtilitarianSmallFlat, along with a new "Utilitiarian Small Flat" checkbox in the "Supported Families" for the target. This doesn't seem to accomplish anything new though, so it's pretty confusing. The very sparse documentation mentions the same watch faces that are mentioned for the standard "Utilitiarian Small". I can't even find any mention of it any of the WWDC talks. "Designing Great Apple Watch Experiences" talks about how all the complication types are used, including the new "Extra Large" family, but there's no mention of "Utilitarian Small Flat". I don't see any mention in the HIG either.
I assume it's OK to just check the box and then return the same CLKComplicationTemplateUtilitarianSmallFlat template for either family?
My best guess is that there are new, unannounced watch faces coming that will only support "Utilitiarian Small Flat", but it's entirely possible I just missed something.
Any ideas?