You are allow to use the license on one machine, if you use it on multiple machine at the time:
1- You'll the the error that license is not valid ( like MS when it tells you license is not "Genuine")
2- You'll be violating the Qlik License agreement.
Now, you may un-register the license and move it along to different machine once you finish working with that client. You just open Qlikview> settings> user preferences > license > "clear license information".
Once its clear you can take it and place it on another machine, but again 1 license : 1 machine.
I would guess the client is a bank institution?