One of the things I like about Turretin on perspicuity is that it's clear enough that the distinctions used are not ad hoc, nor do they arise purely out of polemics (even though polemics is never far away with Protestant scholasticism). You can see this from the fact that several of the distinctions used in the discussion are generalizable to other arenas where clarity or lucidity is important. For instance, I've been fairly critical of the confusions and conflations people tend to make when talking about clarity as a value in philosophy; some of these are highlighted by distinctions Turretin makes.
For instance, when we say that a philosophical text is clear, we may mean either that its content is clear or that the way the content is presented is clear. Indeed, the distinction is crucial, because if you look at some of the things in philosophy that are lauded as 'clear', it's pretty clear that it can't be the content, but only something about how that content is laid out. But sometimes you also find 'clarity' being used as a filter for weeding out not just bad style of presentation but also positions that are themselves thought obscure, regardless of how well they are presented. The term 'clarity' may verbally appear in each discussion, but not with the same sense.
For another, clarity is sometimes attributed to a work only on condition of certain necessary but difficult steps having been taken in interpretation, and sometimes attributed to it on the basis that no difficult steps have to be taken to interpret it. An example of where discussions usually end up being very ambiguous on this point are the common criticisms of Hegel or Heidegger as being unclear: it's often unclear which of these two grounds is met. It's pretty clear that it's difficult to get to the point where Hegel or Heidegger is clear, requiring some discipline, study, and familiarity with particular methods, approaches, and positions. But proponents of Hegel and Heidegger would often say that there is a point where they become clear on key points, however difficult it may be to reach; whereas some of their critics appear to take the fact that you can't just pick up Heidegger and understand right off the bat as a reason for thinking Heidegger is an unclear author; which is a confusion.
And so forth. There are plenty of kinds of clarity that are valuable; but they are not the same, and distinctions need to be maintained. The distinction between formal and effective clarity (the clarity of the work versus the clarity it gives to other things once understood) is one that I confess that I myself have sometimes not made as I should.
