Every cash table session in PokerTracker 4 is defined as a specific limit, which is detected automatically from the hand history files as the hands are imported.
Limits look like "$2/$4" for a full ring fixed limit table with $2 small bet and $4 big bet ($1 and $2 blinds), or "$1 NL (6max)" for a 6 seat NL table with a $1 big blind. Information about antes and other tags such as "Cap" or "Rush" are included where appropriate too.
However, not all sites include all of the required information in the hand histories. In particular a lot of sites do not specify the maximum size of the table, only the number of active players. This can lead to sessions being classified incorrectly.