I don't see these terms in my immediate notes, but REL may stand for machine reliability or Uptime, where you account for any typical downtime at a workcenter. The FPY probably stands for First Pass Yield, which accounts for defect percentage. These are to be factored in against observed machine cycle time, to get a process takt time which you compare against the Customer demand takt time requirement. The highest process calculated takt time is typically your bottleneck or constraint.
For example, if you have a raw machine time of 30 seconds, uptime of 90% and yield of 95%, you would divide 30s/.90/.95, for a takt of about 35.1 seconds, which goes below the process box along the lower row of that staggered line at bottom of the map...