I am reading some lecture notes (the context is the Vlasov–Landau equation but this is irrelevant for this question) and the phrase came up that the equation was "closed by Maxwell's Equations". A quick Google search shows me that this is quite a common phrasing. I can't, however, seem to find an explanation of what it actually means. Therefore my question is this:
What does it mean for an equation to be closed by Maxwell's equations?