My impression in Moab is that the patches are invoked when the traffic comes from links to old conventional VHF repeaters, but I'm not 100% sure. I see the patches linked to TGs that are simulcast with VHF repeaters. Sometimes my BCD436 shows both a standard (4-digit) and a patch (5-digit) TG, but never more than one of each. Usually my scanner only shows one or the other. I haven't figured out what function the patches serve, and linking to more than one standard TG would be a possibility.
DSD+ often lists the Patch/Simulcast Supergroup/Subgroup links when they're established and when they time out. I suppose we could check them at a given time and see how many Subgroup(s) are linked to a Supergroup.