For questions relating to 'flanking' mechanics, by which being on both sides of a target entity provides some sort of effect.
Flanking is generally used to refer to the sometimes complicated flanking modifier rules in the various editions of D&D and other similar systems, like Pathfinder. It may also be used to ask questions about similar rules in other systems, where the term is applicable. Generally, if the rulebook refers to something as 'flanking' and your question is about that thing, this tag is an acceptable inclusion. If not, you'd be better served therefor by the inclusion of one of the other, related tags, like attack-of-opportunity or tactics