"X" the first letter in an acronym, such as in "XR" usually stands for "Experimental": But, when the "X" can be dropped, replaced by another letter, such as "H"for "HSS" (standing for homo sapien sapiens  classified as HSS, meaning an equalifycated* 
subject to being selected for service as a Space Migrant.  

* Definition:- As far as an artificial or living intelligent life form [ILF], determined to have been & widely accepted, as approximately equal  to each other, as indicated by an "H", such as in "HR", are an equalifycated human-robot.