Hi, not staff, but for future reference, this is an auto tag that is added to take care of plural words and add the singular word form of them. They can be seen as the group of tags after the main bunch.
"Sleeve" is almost never a tag by itself. "Sleefe" comes from "sleeves", which is a plural word. The singular form of words ending with "-ves" is "-fe" so that's why "sleefe" is auto added. (Another example would be "knife" and "knives" or "glofe" and "gloves" )
Pretty much all plurals and singular are auto added. "Ships" auto adds "ship" and all words ending with -s and -es have the -s taken off ("capricious" and "capriciou", "legs" and "leg" ). Just in your example, "mistletones" is the name of the RIG the item came from and auto adds "mistletone."
Another type of plural are words ending with -a. "Gaia" and "fuschia" auto add "gaium" and "fuschium" which are not words (-um is the singular form, for example, "bacterium" and "bacteria."