Peter David, writer for the Star Trek comics, wrote in rec.arts.startrek what were acceptable sources for the stories and what not in 1990:
"This is what we have been told by Paramount as being "official/real/ canonical" whatever phrase you want to use, Star Trek.
First Season STTOS
Second season STTOS
STTMP
STTNG
That's it, in terms of what is absolutely acceptable.
The animated series is verboten. No mention to be made, no characters to be used. Nothing. Nada. If it was established on the animated series and we try to use it, we're told it wasn't established.
As for Treks 2 through 5, and the entire third season, nothing in that is absolute. We are told that "some things" are official (i.e., acceptable) and some things aren't. For example, we tried to have a storyline where Chekov is wishing that he could have his own command and were told this is no longer part of his character. When we cited his semi-conscious stating of "Admiral" as rank in Trek IV as an example that it must still be SOMEWHERE in his mind, we were told that that didn't count. We have never been given a complete list of what is acceptable and what is not.
And that is also first hand from Richard Arnold."
Also...
"No, the cat creature in Trek V was not a Caitian. We tried to introduce a Caitian in the Trek comic and were told Caitians don't exist. We pointed out that the cat creature in Trek V (not to mention an ambassador in Trek IV) sure looked like Caitians to us. We were told we were wrong. Welcome to the wild, wacky world of continuity."