Because interface methods are virtual, it is not possible for a caller to know the full set of values that may be returned from any one call. One implementation of a method may return five values; another may return eight.
The documentation lists common values that may be returned for each method; these are the values that you must check for and handle in your code because they have special meanings. Other values may be returned, but since they are not meaningful, you do not need to write special code to handle them. A simple check for zero or non-zero is adequate.