A user data file is a special key in the metadata service that holds a file that cloud-aware applications in the guest instance can access.

You can place user data in a local file and pass it through the --user-data <user-data-file> parameter at instance creation:

$ nova boot --image rhel-cloudimage --flavor 1 \
--nic net-id=3d706957-7696-4aa8-973f-b80892ff9a95 --user-data mydata.file