I'm currently working some testing for a particular area of an application. I had to write some automated tests for a particular feature but due to the circumstances, this was not easy to do. When I asked one of the other testers about it, he mentioned that the same features exist in a sister application our company produces but isn't documented anywhere (end-user documentation or otherwise). He also said that the feature doesn't typically get tested at all in the sister application and isn't usually tested in the application I work on. Apparently this feature isn't heavily used but removing it would require a fair bit of work so the benefit-cost ratio doesn't work out.

All of this has left me with some questions. Other than "The documentation says so" or "We told the client it is", what usually makes a feature "supported" versus an unsupported feature?