From the publisher's website:
Pathfinder Society scenarios are short, 4-hour adventures for use in Pathfinder Society Organized Play. Though they're designed for Pathfinder Society, they can easily be used for any game that uses the 3.5 or Pathfinder RPG rules.
Pathfinder Society Scenarios are divided into "seasons" of adventures. The first "unofficial" season, numbered 1-28, released between August 2008 and July 2009, used the D&D 3.5 OGL system and is referred to as "Season 0." The first official season, Season 1, used the Pathfinder system and was released between August 2009 and July 2010, and are numbered 29-56.
Beginning with Season 2, first released in August 2010, the numbering system changed to (Season)-(Scenario number). Thus Scenario 2-01 is the first scenario released in the second season.