Apart from the clearly defined database options, Oracle databases may include many semi-autonomous software sub-systems, which Oracle Corporation sometimes refers to as "features" in a sense subtly different from the normal usage of the word. For example, Oracle Data Guard counts officially as a feature, but the command-stack within SQL*Plus, though a usability feature, does not appear in the list of "features" in Oracle's list.[original research?] Such "features" may include (for example):