Radiant Crossover is what RX represents but it's not clear what RZ mean, but it's safe to say the R likely means Radiant. What do you think the Z is for?
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Lexus RX/RX L
The Radiant Crossover, or the Lexus RX, represents the automaker's envisioned future when it was launched at the start of the new millennium. The L suffix is used to denote the larger, seven-seater versions.

Lexus Explains What Each Of Its Model Designations Means
Lexus UK gives us a quick rundown of its model range and the real meaning behind those letters so you have something to talk about at parties.

The Radiant Crossover, or the Lexus RX, represents the automaker's envisioned future when it was launched at the start of the new millennium. The L suffix is used to denote the larger, seven-seater versions.