I think it depends on how you’re running jupyterlab. If you’re running standalone lab-location will be just /, if you’re using jupyterhub then lab-location will be a combination of where jupyterhub is running and a user-id e.g. /hub/username, and in either case if you’ve got a proxy you might choose to run them under a custom /prefix (can’t remember what the configuration properties are).
I don’t use jupyterhub so I take it this is going to be empty for me then;
IMHO it is kind of odd that jupyterlab’s documentation accounts for extra addressing bits that are actually managed by another, optional, layer of tooling ?