A comment’s context is some extra information that is added to the comment by a consuming application, such as P2 Explorer, and is used in a way that makes sense to that application.
For example, in P2 Explorer, the context is determined by the page designer. In the case of the comments button component, all comments entered via that component on a page has the context as whatever is decided by the page designer. To illustrate, suppose the comments on a page all have the context of “Oil Report”. Explorer can apply filtering to all the comments in the system to only show comments relevant to the current Explorer page.
Taking this a step further, a page designer can use the same context for the same entity on a completely different page. When a comment is applied to that entity, users will see all comments for that entity and context (e.g. Oil Report), even though they are displayed on different pages.