TEMP. Help for stock rom selection in xperiafirmware

In xperia firmware 14.5.A.0.270 / R4D, 14.6.A.0.368 / R6D

what is meaning of R1D, R2D, R4D, R6D cant understand

My handset:
customization version: 1276-0875_R2D
Active customization: 1276-0875_R2D
Phone software version: 1274-8313_14.5.A.0.242

please explain about this. Thanks