What "unmetered" actually means

There is no traffic cap on any plan and no overage billing. You will not get an invoice at the end of the month because you used what you bought, and there is no fair-use clause that quietly turns into a charge.

What varies between plans is the port speed.

Why the speed differs by plan

Port speed scales with the plan. Capping each server's port is what stops one busy machine from slowing down the servers around it.

It is a deliberate trade rather than a hidden throttle: you can see exactly what you are getting before you buy.

Resizing

Upgrading changes the port speed immediately, and you pay the difference rather than rebuilding the server.