What are units in the build your own configuration?

Units are just used to measure how much you'll like to upgrade, a fraction of the previous settings.
The unit starts at 16 because that's the configuration of Shells Pro, so any unit higher than 16 will get you a higher configuration than Shells Pro. So if you add 4 more units you'll increase your VCPU by 1. This allows you to pick a RAM and VCPU that you'll like as displayed on the upgrade screen.