Most of the time you will see a number in the margin. Look up that number in this Writer’s Guide and you will find out what is wrong. Sometimes we use one of the following marks instead:

sp - Spelling Problem.
There is a spelling mistake on this line.

c - Capital letter problem.
There is a word that has a capital letter but shouldn’t, or
There is a word that should have a capital letter but doesn’t.

p - Punctuation problem. (.,?!;)
There is a missing punctuation mark, or
There is a punctuation mark that shouldn’t be there, or
There is a punctuation mark that is wrong.

? - Sense?
I don't understand this.
See me if you can't see why.

^ - Word missing.

x - Incorrect.
There is a mistake on this line that we don’t have a special mark for. See me if you don’t see it.

Here are some of the standard editing marks. Click to Enlarge