S89,
Not sure if that is still true. The original plan was X was the KC-135 replacement, Y was the KC-10 replacement and Z was awaiting definition.
Latest stories have Y now being very much an run-on of X, which is why Boeing is rumoured to be pushing for follow on KC-46 orders and Z being either a stealth tactical tanker or a mix of a large MRTT/777 size tanker with stealthy UAV adjuncts (e.g. MQ-25) to ferry fuel forward to the combat zone in the Pacific.
Initial "sought for" request reflects that.
https://sam.gov/opp/df2d170039c44898...a6dac295d/view
"....
The Air Force is seeking companies that have the capability to deliver approximately 140-160 Commercial Derivative Tanker Aircraft—at a rate of 12 to 15 per year—to supplement the Air Force Tanker Aircraft fleet at the end of KC-46A production, and bridge the gap to the next Tanker recapitalization phase. The Commercial Derivative Aircraft must be operational by 2029.
The Air Force is still finalizing the requirements for this acquisition. However, the baseline for aircraft capability will be based on the requirements from phase one of tanker recapitalization with subsequent and emerging requirements as defined by the Air Force...."