System Variables
System variables are created automatically by Labvanced; they cannot be created, modified, or deleted by hand the way a custom variable can. They track experiment-level information that Labvanced already knows on its own: trial number, subject counter, browser type, screen size, session start and end times, and, in a multi-user study, Role_Id.
Why they're useful
Because system variables update automatically, they're particularly useful inside a Requirement Action, to branch behavior based on something Labvanced is already tracking rather than a value you'd otherwise have to record and maintain yourself. A common pattern is executing an action only on a specific trial number, or, in a multi-user study, only for a specific participant.
Read-only
A system variable's value is set by Labvanced itself, not by a Set / Record Variable action. They can be read anywhere a variable can be read, in a Requirement Action's condition, in the value-select menu of another action, or in the exported data, but not written to.
Further Reading
System Variables Table
The full list of every system variable Labvanced creates automatically, with what each one tracks.
Role_Id
The system variable that identifies each participant in a multi-user study.
Variable Types
All six variable types in Labvanced, system variables among them, and when to use each.