I imagine this would have been easier to determine if the commands were documented on the wiki :D
It looks like the APM config guide has the wrong name for this event:
https://support.f5.com/kb/en-us/products/big-ip_apm/manuals/product/apm_config_10_1_0/apm_config_irules.html122471
ACCESS_SESSION_START
This event occurs when a new user session is created. This is triggered after creating the session context and initial session variables related to users source IP, browser capabilities and accepted languages.
Using ACCESS_SESSION_START
This event provides a notification that a new session is created. You can use this event to prevent a session from being created when a specific event occurs. For example, if the user is exceeding the concurrent sessions limit, or if the user does not qualify for a new session due to custom logic, you can prevent a session from starting.
You can use ACCESS::session commands to get and set various session variables. Admin can also use TCP, SSL, and HTTP iRule commands to determine various TCP, SSL, or HTTP properties of the user.
Aaron