As your applications move through your workflow into your reviewer process, you can set it up so that when applications move to the reviewer stage, they are available to your selected reviewers. This process is configured within the Assignments tab of your review stage settings.
These assignments are automatic and distribute all incoming applications that enter the review stage to a set of reviewers and/or review teams.
If this process is configured, as applications move into the review stage they will be randomly, and evenly distributed to your selected reviewers.
Custom Rules are useful if you have groups of different applications and they are to be assigned to different reviewers/team. For example, if your applications are divided by region and a different team of reviewers is to review their corresponding region.