Register is a term used in musicology to mean a range of notes an instrument can play, except that here we are looking at register within language use, and especially within performances of writing and reading. For instance, the spectrum from the concrete to the abstract and all steps in between would constitute an "octave," and so would other registers that constantly are at play.
I suggest that the practice of distinguishing register at work in a given text would also allow us to distinguish how we project and perform with a particular register whenever we read or listen, and of course, when we write. Once you can distinguish registers, then you can "hear" or "see" them in actual texts. In fact, you might even consider that registers do not exist per se except in those moments when we distinguish them. You can't catch what you can't see, and you can't write what you can't read.
I have adapted the following four registers (ranges) of language use from Tzvetan Todorov's Introduction to Poetics, pages 21-26:
Keep in mind that when you write or evaluate writing, any position along the range of a given register has its value while at the same time poses a problem. It is important to examine both aspects of each register in a given piece of writing.
The polyvalent register comes alive for the reader once the reader has begun to explore the "traces" both iterated and presupposed in a given text (see James Porter's "Intertextuality and the Discourse Community"). The role a reader must play to experience the polyvalent register is what Peter Rabinowitz calls the "authorial audience," the audience the implied author addresses (see "Truth in Fiction").