Is it just me, or is this tag heavily misused? IMO, alternate costume does not mean alternate clothing. A costume is clothing that is distinctive of a class, profession, ethnicity, nationality, activity, etc, the keyword here being DISTINCTIVE.
I pulled a couple of examples for posts that I believe do and do not fit this definition that are tagged as such. I used the same character, Konpaku Youmu, for simplicity in comparison.
Search terms: Konpaku_Youmu Alternate_Costume
Alternate Costume
post #2342108 (Kimono, distinctive Japanese dress)
post #2335505 (Maid, distinctive profession uniform)
post #2323134 (China dress, distinctive Chinese dress)
post #2293875 (School uniform, distinctive activity uniform)
post #2272630 (Kendou gear, distinctive activity uniform)
post #2341747 (Lolita fashion, distinctive style of dress)
Not Alternate Costume
post #2363732 (Casual,Contemporary)
post #2363326 (Contemporary)
post #2277665 (Adapted Costume)
post #2322202 (Adapted Costume)
post #2346859 (Adapted Costume,Contemporary)
Adapted Costume
Those last three examples bring up another important distinction. Correct me if I'm wrong, but I believe that alternate costume and adapted costume are mutually exclusive terms. Adapted costume is like having different shades of the same color, whereas alternate costume is like having different colors altogether.