R and Z are both revenue First/BF fares. R is "cheaper" (though there may not always be both an R and Z fare on the same route), and does not earn the 50% class of service bonus miles (though it does earn 150% EQMs).

For example, there was a recent thread about a $2400 R fare EWR-PEK in mid-February (e.g., President's day weekend). And I think all the winter holiday special BF fares to Europe book in R this year, not Z.

There were also some changes to the award inventory buckets. They are discussed in more details in a thread, somewhere in this forum...surely you know how to search?