In Statsig, user allocation to a holdout group is a predetermined process that occurs automatically when the holdout group is created. This allocation is based on a cryptographic technique using a salt, which is applied to the unit_id (userID or stableID) of the user being evaluated.
As a result, the assignment to the holdout group is stable as long as the user retains the same unit_id. There is no specific trigger required for this assignment to take place. However, it is important to note that while the assignment is predetermined, the actual exposure of a user to a holdout group only occurs when the user encounters a feature gate or experiment that is part of the holdout.
This means that Statsig only becomes aware of the users when they are exposed to a gate or experiment. It is also crucial to understand that if you are managing the assignment for holdout yourself, you will need to manage exposure logging. This approach is not commonly recommended for holdouts due to the risk of polluting the holdout data.