Written for the second annual mini meta fest
The thing is, I'm probably not the right person to write this, but I will anyway. For me, fanfic is divided first and foremost by fandom. I know some people will read things for fandoms they don't know if they like the writer enough, but I can't do this.
Secondly, it's divided into shippy and not shippy. Fic can of course be very plot driven but still have romance or an established relationship in the background, and then it will still be considered not shippy by me. Shippy fic is the kind where romance or sex is the point of the fic.
Last comes the distinction of het, guyslash or femslash. It's not a really important distinction for me, since I will read and write fic of all kinds, but I know it is to others, and in some way I think marking something as femslash is important, to show that we exist, that femslash exists. I've recently begun tagging my femslash on AO3 to make it easier to find, and to help the statistics (if someone was going to make such statistics). But to me personally it doesn't mean a lot in terms of reading.
If we're just going to look at the word, femslash to me is fic in which two (or more) characters who see themselves as female are in a romantic or sexual relationship, or are on the verge of entering into such a relationship, or if one character pines after (for?) the other. If one or both (or more) of the characters are in a straight relationship at the time, or have been, or will be, or indeed any other kind of relationship that can't easily be labeled as lesbian it will still be femslash. Sure, if it focuses a lot on the straight relationship it might not be very good
femslash, but still.
The only exception is poly relationships, at least if the focus is on that aspect of the relationship and not the characters as they relate in twosomes within the poly relationship. And this only applies if the characters are of two (or more) different genders.
All in all, I do think it's inherently problematic to divide things according to sex or gender, but while I wait, and work for, a society in which sex and gender is irrelevant I will continue to label things when appropriate.