These disc-shaped tokens are used in Esmer shuttles, which can't take off without one of these.
When Twinsen is escaping from Zeelich to Twinsun, he needs to get a token in the tower of the astroport in order to take off in a shuttle. The same situation is presented when Twinsen needs to use the emperor's shuttle to go from Island CX to Otringal.
"The travel token allows to program the destination of an Esmer shuttle. Without it, the spacecraft cannot take off."