70-persistent-net.rules is a generated file that remembers NICs by MAC address and/or where on the bus they’re connected while 75-persistent-net-generator.rules is a static file that generates and updates it.
For an example, When new NIC is discovered, 75-persistent-net-generator.rules names it and adds a rule to 70-persistent-net.rules that will give it that name automatically each time it’s enumerated.
70 has a lower number than 75 in order to run first. 70 will catch any NICs it recognizes, then 75 will take anything that hasn’t already been named and generate a new name for it.
This is quite nice, if you’ve assigned static configs to a bunch of NICs and then changed the hardware configuration so that they would normally be renumbered. It’s also a lifesaver if you have one of the rare weird hardware configurations that doesn’t enumerate in the same order every boot. However if you replace a NIC, the new one gets a new name so you have to update config files.