cloud-config / cloud-init : standard variables / custom variables

[ITJamie]

Is there documentation anywhere of the list of available variables for cloud-init/cloud-config templates?
Is it possible to use custom variables from a custom vm template in the cloud-init/cloud-config template?

[olivierlambert]

The official Cloudinit documentation for example? https://cloudinit.readthedocs.io/en/latest/