When a character or span of characters is missing outright, it is preferable, though not always feasible, to register this fact with a <GAP>. Our practice hitherto has been to distinguish between entire missing pages (or sometimes partial pages, e.g. when a page is torn in half) and individual missing letters.
We capture the absence of missing pages using <GAP DESC="missing"> Exact EXTENT values cannot always be supplied, but one can almost always supply a minimum value, e.g. EXTENT="2+ pages". In may cases, pagination or quire signatures can provide enough context to be more exact, e.g. EXTENT="14 pages", etc.
Individual characters, or spans of characters, if their absence is noticed, should be captured as if they were illegible characters. That is, the keyers should capture the absence as "$" or "$$" or "$span$" or "$word$" (etc.); the reviewers should convert these into <GAP DESC="illegible" EXTENT="1 letter"> (or "2 letters" "1 span" etc.). If a REASON is supplied, it can be "missing" or "page cropped" etc.
Though we prefer that vendors notice missing letters and record them as illegibles, this is not something we can reasonably insist on, e.g. in the following passage, the last two lines clearly lack an initial "I"; the keyers did not register this absence with $, but they should not be penalized for that.
Deliberate blanks may be of many kinds.