Tag:informal=no

informal=no indicates that a feature has been established on purpose and that it is formally set up. It is less commonly used than its opposite tag, informal=yes, which is often used to indicate a feature that is not formally maintained but exists because of repeated use, such as a social trail (highway=path + informal=yes) or an unofficial tent site (tourism=camp_pitch + informal=yes). However, informal=no can be useful to state that a feature is officially established, e.g. when the appropriate operator=* value is not known or if you would expect from the context that it may not be formally established.

If the operator of the feature is known, it should of course be tagged explicitly. The presence of an operator=* tag does not imply informal=no, as the operator tag is about the actual (de-facto) operator, which may be formally established or not.