Convenience of Environments -artificial intelligence:

In some cases, some aspects of an environment which should be taken into account in decisions about actions may be unavailable to the agent. This could happen, for instance, that's why the agent cannot sense certain things.  These cases, we can say that the environment is little bit inaccessible. In this case, the agent may have to informed guesses about the inaccessible content in order to act rationally.  

The builders of RHINO discuss about "invisible" items that RHINO had to deal with. These included glass cases and bars at a range of heights which could not be detect by the robotic sensors. These are clearly inaccessible aspect of the environment, and RHINO's designers took into account while designing its programs.