- rugby: for general questions about rugby which cross both union and league
- rugby-union: for union specific questions
- rugby-league: for league specific questions
As an example of a question which I think would be improved by this, see Rugby penalty kick into touch-in-goal - that's absolutely a union-specific question, and the answer will be significantly different for league. Yet the only thing we can tag it with is rugby which includes league as part of its definition.
See also: Should the Rugby tag be removed - replaced with just Rugby-Union and Rugby-League?, although that's from 2012.