ABBREVIATIONS
BTS. Behind the Scenes. Meaning the character's presence was felt, but he/she wasn't actually seen. This has an extremely broad range. I usually only cover if they were obviously doing/causing something, but just not seen.
[word]. Under bibliography, a title in brackets indicates it is a Behind the Scenes appearance.
(?). A question mark in parentheses means the word or phrase following it either is in question, or doesn't make sense to me.
??. Double question marks, usually following a first name, means the character's last name is unknown.
xd. extra-dimensional. not native to Earth's dimension
xt. exta-terrestrial. not native to Earth. (all xd are xt, but not necessarily vice-verse. analogous to the square and the rectangle)
OHotMU. Official Handbook of the Marvel Universe. Refers to the reference comic books.
UOHotMU or UHOTMU. Unofficial Handbook of the Marvel Universe. Refers to the website styled after the previous books. http://www.marvunapp.com/handbook/
Otherwise, I try to write things out. If I abbreviate anything in the bibliography, its usually because I've spelled it out earlier in the entry.
Let me know if there are others.