Exformation is a term meaning explicitly discarded information...I see at least two problems with this definition. The first is that there is already an agreed-upon linguistic term for the kind of information identified here as "exformation." It's called "given information," i.e. "information that is assumed by the speaker to be known to, assumed by, or inferable by the addressee at the time of the speaker's utterance, because it is common knowledge, part of the extralinguistic context, or previously established in the discourse."
Effective communication depends on a shared body of knowledge between the persons communicating. In using words, sounds and gestures the speaker has deliberately thrown away a huge body of information, though it remains implied. This shared context is called exformation.
Exformation is everything we do not actually say but have in our heads when, or before, we say anything at all - whereas information is the measurable, demonstrable utterance we actually come out with.
If someone is talking about cows; what is said will be unintelligible unless the person listening has some prior idea what a cow is, what it is good for, and in what context one might encounter one. From the information content of a message alone, there is no way of measuring how much exformation it contains
The second is that the short definition -- "explicitly discarded information" -- doesn't seem to match the extended definition and examples offered here. Assuming that somebody knows something about cows and then neglecting to mention those things is not explicitly discarding it. It's precisely the opposite: implicitly assuming it. Unless you say to your conversation partner, "I know you know many things about cows, so I choose not to mention those things now."
A better sense of "explicitly discarded information" would be to look at part of the body of information that is generated but then cancelled before some final state -- say, a researcher's notes which aren't included in the paper, an idea or an argument you could make in conversation but choose not to -- in other words, the full background of information which potentially would be relevant and can not be assumed to be common knowledge but which is nevertheless absent or cancelled to give greater emphasis to the information with greater force or value. And I think this is what Kottke might be referring to when he says "the more exformation you generate, the better your writing, design, art, photography, or blogging will be."
If you write ten pages every day and keep only your best paragraphs, the rest is exformation. Knowing a great deal about higher mathematics while teaching high-school algebra is exformation. (Most of teaching and teacher education boils down to managing exformation.) It's separating the piths and the gists, the luminous particulars, from the stream of information. It's the part of the iceberg beneath the surface, the part that gives what's shown its power.