Sections at the Rogers Centre are split down the middle by an aisle. Seats on the right side of the aisle are often have their section name suffixed with an "R" and seats on the left side with be suffixed with an "L". In section 139, for example, seats 101-110 are on the left side of the aisle and seats 1-11 are on the right side of the aisle. You'll also find that seat 10 in section 140/140R is directly next to seat 110 in section 139/139L.
Sections with these L/R delineations are typically referred to as "aisles" since they share the same aisle/stairway. Modern venues rarely use this system to organize their sections as it can be confusing to fans. On RateYourSeats.com, we do not separate R/L sections.
Some sections may also be labeled with an "A", "B" or "C". This is simply an arbitrary label. Section 113BR is simply the right side of section 113.