An obvious but nonsensical, answer would be “all matters.” That cannot be the solution since it begs the question “What makes something a matter?” (See my post of March 26, 2008: definition of the term “matter”.).
Many law departments deem a set of legal activities a matter for purposes of tracking them on a matter management system when the law department retains outside counsel to assist. To keep track of how much goes to each firm, and to all law firms in general, and perhaps to charge those fees back to a business unit or staff function justifies creating and updating a matter.
More of the work of a legal department finds its way into a matter management database if the department defines matters to be tracked along the lines of those that are “significant” or “more than four hours of work” or some other criteria.
Another possibility is to let practice groups define what kinds of matters they wish to include in the system. Employment lawyers might filter and clump their activities differently than would lawyers for the widget business unit. I have not seen a variable rule like that but it would have the advantage of being tailored to perceived needs and therefore more likely to be sustained.
Law departments that track time end up with some “general” matters for hours that would not otherwise have a matter for a home. And, one final thought: it should not be assumed that all matters in a system should have all the same fields populated. The more important the matter, the more fields should be the operative rule.