People within GM and the industry use both the L (Longitudinal) and R for that transmission series as old habits are hard to get rid of.
GM / GM GPS (Global Propulsion Systems) [newer name for GM GPS] are using 10Rx0 for that series now in correspondence but are not consistent.
Here's the official GM GPS USA Information Guide for 2017 MY, the launch of the 10R90 in the ZL1 as we all know. I catch myself in emails, posts, etc. still occasionally using 10L90 for example. I just did a little cut & paste from it. Below it is the link for the 2018 where it and the 80-series are called 10L80 and 10L90. So, I'd say it's fair that both are "right"...
https://gmpowertrain.com/assets/docs...tion-Guide.pdf
https://gmpowertrain.com/assets/docs...tion_Guide.pdf
2017
Chevrolet
Bolt MMF 1ET25(MMF)FWD
Camaro LTG 8L45(M5T)RWD TR3160(M13)RWD
LGX 8L45(M5T)RWD TR3160(M13)RWD
Camaro SS LT1 8L90(M5U)RWD TR6060(MM6)RWD
Camaro ZL1 LT4 10R90(MGL)RWD TR6060(MJK)RWD
2018
Chevrolet
Bolt MMF 1ET25(MMF)FWD
Camaro LTG 8L45(M5T)RWD TR3160(M13)RWD
LGX 8L45(M5T)RWD TR3160(M13)RWD
Camaro SS LT1 8L90(M5U)RWD TR6060(MM6)RWD
Camaro ZL1 LT4 10L90(MGL)RWD TR6060(MJK)RWD