We don't use NFS and haven't for a long time since all of the clients are Windows. We use SAMBA for file sharing.
I have used NFS in the distant past when we ran a number of SUN and other Unix workstations, but it's been years and I've forgotten much and NFS has evolved.

I have recently considered using NFS to share files between servers and had a question about default NFS access.

Does Ubuntu NFS limit access to an exported file system in any way?
Does Ubuntu NFS require user authentication to access an exported directory?

For example, who could access this file:

/file *(rw,sync)
Would this require the user to be a actual system user?

Is there a ufw command to limit access to the local network?