A your character here commission is a special type of commission where the person who offers it (the artist) provides an unfinished sketch of the final artwork.
During the course of a YCH you can bid for a to be finished version that will then feature your character of choice in the final piece. The character will still feature the same pose and expression that the character rag doll in the sketch had.
YCH auctions have the big advantage of the buyer not being required to come up and define a pose, that he then has to explain to the artist. Instead, he can just provide the artist with a character reference.
A character auction is just auctioning the character itself. More often than not, the owner of the character will provide additional artwork with it - but it is no requirement.
If you’re bidding for a YCH you need to provide the character for the finished piece. In a character auction you are bidding for the character itself.