It's OneFS FEC protection nomenclature for our "hybrid" protection polices. More generally, it's written as:
N+M/b
where "N" is the number of data stripe units in a stripe, "M" is the number of FEC stripe units in a stripe, and "b" is the number of stripe units in a stripe which are allowed to reside on the same node.
Reference: https://www.dell.com/community/Isilon/Isi-get-output/td-p/7085467