Note: definition of this tag varies upon team.

For some teams this is used as a generic tag to mark a task for projects without a defined ending date but which can be definitely realistically be defined as finished at some point.
For other teams (Web-Team) this signals a task that is S.M.A.R.T (Specific, Measurable, Attainable, Relevant, and Time-Bound)

See also Tracking-Neverending tag which serves for marking older tasks which will never be "fixed", but are instead used for grouping of cross-project related issues by given feature or attribute - the Tracking-Neverending tag should not be used; instead project tags should be created and used instead.