regionals are like SWUs but for a more restricted set of flights with the main advantage is conformation at booking or before complimentary upgrades. Also good for non-elites
Presently for UA -- North America including Caribbean and IAH-LIM. For CO North America plus Caribbean and Central America, and select flights in Hawaii and Asia/Oceania. In the future see Upgrade regions
So Caribbean is a go and the regionals can be used on mixed metal flights. You need to call and ask to use CO regionals on UA metal and vice versus -- I just upgraded a CO transcon with UA regional today. (BTW for the next 3 months, best to CO status on CO metal and UA status on UA metal)