On swap-bot, all members have a profile where they say what they like and don't like. Some swaps are then based on the profile of your partner: you have to send mail-art (or ATC) that will fit to the "like" list of your partner. For the last swap of this type that I participated to, both of my partners have selected science-fiction in my list of hobbies and they sent me these two wonderfull ATC.
First Stella Van Domburg from Neitherlands picked up my favourite movie (Forbidden planet).
And Dawn Edwards made this fantastic "doodle" ATC. I also share with you the very nice envelope that she used to send it.
In the profile of the partners I had to send to I picked up the fact that one likes gerber daisies and the other likes imaginary animals like unicorns.