The scope of an article is the topic or subject matter, which is defined by reliable sources. The extent of the subject matter identifies the range of material that belongs in the article, and thus also determines what does not belong (i.e., what is "out of scope").
The lead, ideally the introductory sentence or at least introductory paragraph, of an article, should make clear what the scope of the article is.
Editors are advised to stay on topic, and to ensure that articles contain no irrelevant (nor only loosely relevant) information.
The two main reasons for splitting material out from an article, are size and content relevance. If either the whole article, or the specific material within one section becomes too large, or if the material is seen to be inappropriate for the article due to being out of scope, then a split may be considered or proposed. Consideration must be given to size, notability and potential neutrality issues before proposing or carrying out a split.