When specifying an NFS mountpoint in /etc/fstab, What is the "addr" option for?

It can specify the client address to use when the client has multiple subnet IPs on an interface and traversing the same routes (think aliased eth0:1, bond0:2, etc.). Typically in this situation you'd put the NFS server on another interface/subnet to avoid the whole problem but in large network installations things can get messy.