How does the system decide Job Alerts?

Summary:

We have many questions from candidates regarding Job Alerts. The system appears to only send three at a time. But other than that it is not clear how it decides on the three. Also users who sign up for Daily often do not receive any?

Content (required):

How do job alert notifications actually work? We've recently had reports from candidates that they have never received an email notification despite signing up for them. How is a notification triggered - is it automatic when a requisition is posted or does something need to be done by the recruiter? We allow users to indicate the "areas of interest" as Catagory and Location? Then they see jobs posted that match but are not send in thier Job Alert. I cannot find anything in any of the configuration or user guides about this. Grateful for any help even if