In the Nasuverse there is one place people can meet with out causing the world to you know go boom. In this cafe people who should never be able to meet each other can talk and get a cup of coffee. It shows up in every city though at a different location for each one.
Though the cafe has it's roots in the Tsukihime world, Lancer and a few others work part time here as well. This is where the BAA gets it's food, where the Neko copies of certain characters are born from and where two teenage girls happen to work at.
Basically the cafe is the setting for Mahou Tsukai no Hako along with All Around Type-Moon! The cellphone site for T-M and the gag cross-over comic in T-M Ace. Hibiki and Chika work part time at the cafe, they also have a tsuntsun fangirl who goes in to the cafe every day to see them. They're waitresses and they talk to everyone who comes in, meaning they know all the T-M characters. Though they only know them as people who go to the cafe often.