Can these be used if the splitter is in a plastic box outside?

Yes and No......keep in mind the splitter is not weatherproof even in the box outside. So corrosion can happen and cause you to replace it occasionally. The termination caps help seal unused ports from letting signals leak into and out of the system causing interference. They also help reduce reflected signals at the end of unused ports. The idea has been around for many years but CATV providers are now using much higher frequencies and the termination caps help kind of button up the cable system form outside electrical noise which can cause them trouble. Good question. Ron - ron.kipper@idealindustries.com