UI are just a form of communication after all, and could be thought of in terms of protocol implementation. A mentor (of sorts) years ago in a standards working group addressing communications for support systems proposed that a comm. protocol design should consider four aspects. (1) Syntax specify symbols and their arrangement. (2) Semantics specify how symbols are interpreted. (3) Intra-message context specifies the 'rules' for using symbols within a specific message format(i.e., reg. expressions). (4) Pragmas (otherwise called "Shared Knowledge"), this is the toughest to tie a meaning to, and yet is the essence for effective communication. Basically it deals with commonality of experience. This is the point to which above the discussions point and contains the answer to the original question. This is the principle that the UI designer is designing to... . In other words, ask the potential users what they what to do!