Use this tag when examining what is on-topic or off-topic, and why.
scope is used to define what is pertinent to discussion. Generally, the scope determines what the site is about. Issues arising from scope might have to do with whether a certain question is on-topic or not, but also can be used to evaluate whether an entire topic should be up for discussion (especially as it pertains to specific Stack Exchange sites).
Using this tag indicates that the question is requesting help with scope (normally as a specific use of a general topic), and answers should generally relate to the scope of the site and whether the topic raised generally relates. It also may be the case where a user has a question, but doesn't know where it belongs.