This is where you communicate with people who you want to collaborate with or asking for someone to collaborate with you.
If you want to collaborate with someone on a certain part, you would say (for example):

Hey Foxy_Skipper , do you want to collaborate on part 23?

(Sorry Foxpai XD)
And the artist you're asking can say no without penalty. (Or you getting angry at them)
If your fellow artist says yes, however, further production of your part is completely your responsibility. I don't care what you do as long as it's in on time.