*BYOC means "Bring your OWN chair".
When a party invite says, "Bring something to sit on" or "Bring your own chair"--it would be wise if you did. Because even though you think, "Oh I'll just sit here for a moment" or "There's enough blanket to share"--the owner might have other plans. (Or maybe the owner doesn't wanna look like a douche bag by trying to be nice about TRYING to get their chair back.)
Such as: Using the chair they brought for themselves.
Sharing the blanket with a significant other.
Bring YOUR OWN chair for YOUR OWN use!!