'Femme' is an odd category in Transformer society. Officially it's a subgroup like cars, Minibots, or fliers, but unlike the other categories, it rarely stands on its own... in appearance femmes tend to be sleeker and less angular than other subgroups, with little in the way of visible kibble, but there are bulky angular femmes and sleek small non-femmes so you can't really define femme-ness by that.

In my writing I tended to treat femme as a class - usually meaning speedier, stealthier, more agile, more geared toward infiltration and hand-to-hand combat than heavy-load weaponry or brute force attack. Their alt modes tended to be sleeker as a result. That meant I had a lot of fun playing with the gender identity of 'bots like Blur and 'cons like Strika, who go greatly against "the norm" and might even be considered a- or trans-sexual by human standards.

Of course, I'm anti-Quint origin, pro-Primus and believe in TFs as beings that would cohabit mech-to-mech as easily as mech-to-femme or femme-to-femme, so your mileage may vary ;)

(If you're interested, I tackled the topic here and here and here, back in the days where I wrote for more than just work purposes.)