When you create and print a box label, the label will contain a QR code image that will be a link to the IMPS box contents page for that box. This QR code is generated by IMPS based on your IMPS server's IP address. This means that your IMPS server must have a static IP address in order for the label QR code to work correctly.
In addition, if you use Flask to test IMPS, the QR code will point to the IP address of the server, plus to port on which Flask is serving the pages.
For example, if your home server's IP address is 192.168.0.1 and you start IMPS with Flask's default settings, IMPS will be served at 192.168.0.1:88 and the QR code on your labels will also point to that IP address an port. If you later move to using a WSGI server that serves IMPS on port 80 (the standard HTTP port) the labels will no lponger work, because they will be pointing to the old port.