You asked:

Can this thing work on any model, or do you have a single model that it runs on?

Any model, however It is possible that the character mesh may need to be somewhat similar to the characters I use, I have used about 3 different characters on the biped model so far with no major problems. I am aware of a issue that can occur on the shoulder if the mesh is not done correctly, but it is not a difficult issue to solve, it mostly occurs whoen you try to use a very high count polygon mesh, and most people are unlikely to do that.

Do you have to specify, like, what faces are the knees, and which ones are the elbows, neck, etc?

No . . . . esentially, I put the character in the scene, facing the plus Z direction, hit a button and the skeleton comes into the scene. Then I adjust the skeleton to fit the character, (head, hands, fingers, feet, ect . . . the skeleton is scaleable, you can scale the arms, legs or whole body by pressing buttons) Then I hit a button that adds "stabilization" . . . it basically just eliminates a lot of skin weights painting. After that I skin the character to the bones, and the character can walk at that point by hitting the play button. I have one other step that I do before being fully ready, and that is setting up the eyes so they work with sliders that are contained in the controls of the program.