Dunno anything about RIA but yes, shotgun pellets are rated at different sizes. The sizes are determined by numbers, higher numbers = smaller pellets. Bird shot ranges in size from a 9 to a 1, depending on the game you are going after. The next bracket is then Buck where it starts at 9 down to a 1, where 1 is the largest buckshot pellet size.

Edit: Buck is a special case because the pellet size gets even larger after 1 with 00 and 000 shot. https://www.wideners.com/blog/shotgun-load-types/

Dunno anything about RIA but yes, "light load" vs "heavy load" is based on the shot size. Shotgun pellets are rated at different sizes. The sizes are determined by numbers, higher numbers = smaller pellets, which is a "lighter load". Bird shot ranges in size from a 9 to a 1, depending on the game you are going after. The next bracket is then Buck where it starts at 9 down to a 1, where 1 is the largest buckshot pellet size.

Edit: Buck is a special case because the pellet size gets even larger after 1 with 00 and 000 shot.

https://www.wideners.com/blog/shotgun-load-types/