Comment 3 for bug 2046490

I agree that it is confusing, but it is spelled out in the top two sections of https://cloudinit.readthedocs.io/en/latest/reference/network-config-format-v2.html .

In particular:
"Cloud-init’s support for Version 2 network config is a subset of the Version 2 format defined for the Netplan tool."

and

"If you are writing or generating network configuration that may be used on non-netplan systems, you must limit yourself to the subset described in this document, or you will see network configuration failures on non-netplan systems."

We're open to suggestions that could make it more clear though. The 'Give feedback' button at the top of that page would be the easiest way to file a documentation issue.