Mies van der Rooh's pass is his identity card. Just like Twinsen's, it has his identification number (0081) and a photo of him, along with a FunFrock symbol.
After finding a note from Mies in the safe in FunFrock's headquarters, Twinsen heads to Mies' house and talks with him. Mies tells Twinsen to pick the pass from his desk, which grants him acces to the teleportation center.
Inventory Description[]
"M. Mies van der Rooh's pass. This pass will grant you access to the Teleportation Center. You will still have to use Gawley's Horn on the left, after the first guard post."