A tag is a unique textual identifier for an object such as a node or a pattern.   Tags are user-defined and can be used as descriptive comments.   Maximum tag length is 50 characters.   Tags are case-sensitive:  for example, the tags "SearchText" and "searchtext" are distinct tags.

A tag must uniquely identify the object it tags (there is one exception to this rule: node tags).   Therefore, once a tag has been chosen for an object it cannot be used to tag another object.