I thought it might have something to do with the amino acids. Valine is interchangeable with methionine so I thought it might represent that. Not 100% sure though.
Having edited the OP to correct the display format: the + sign in this case means functional equivalence (even though those two AA are not identical as some others in that alignment). @decosterwouter responded when the sequences looked as if they were tab formatted blast output.
They are the the same general class and can possibly replace each other (context would be important) and not affect the overall function/structure. Remember the AA substitution matrices (PAM, BLOSUM etc).
I thought it might have something to do with the amino acids. Valine is interchangeable with methionine so I thought it might represent that. Not 100% sure though.