Abbreviations

Comments

?: Indicates something that is not clear. ?? indicates something completely baffling.
Circled or boxed: Circled or boxed text has a comment or instruction in the margin.

For grading only, not typesetting markup:

underline: Indicates important text, usually with a check, squiggle, or X; the crux of the answer being graded.
: Indicates semi-important text, not as important as underlined text.
'PARTIALLY': Partially true, or only part of the required answer.
: Indicates something good.
(): Indicates something somewhat good, but not what it should be.
: Indicates something questionable.
×: Indicates something bad.

∧: Indicates where marginal or interlinear text is to be inserted.
: Indicates a space.
or close up: Indicates extra space that should be closed up in a word (the example should be "close up") or between words.
: A double underline indicates lowercase text that should be uppercase, or uppercase that should be lower (the example should be changed to "Double underline"). (OK, I guess this is non-standard — Merriam-Webster says triple-underlined. Oh Well)
interlinear text: Interlinear text written in, over, or under the text being proofed should be inserted where it appears (or at the nearby ∧ or ∨).
: Indicates text that should be moved over, usually to align with something else. Can point either left or right.
: Indicates where text should be opened up with a space (the example should be changed to "open up").
: Start a new paragraph here.
: Start a new section here.
: Indicates two phrases whose order should be inverted (the example should be changed to "after put these").
Strikethrough: Struck-through text should be deleted.
Underline: A single underline indicates plain text that should be italic, or italic text that should be plain (the example should be changed to Underline).