Hello Debdoos,
Applicants on the reserve pool are made guaranteed offers once a place becomes available. There is no order to the reserve pool which is active until August before September start. We would usually ask the Head of the department to confirm the names of reserve applicants which they would like to offer a place to next, and sometimes that involves going through the report forms of those on the reserve pool.
Once a name(s) has been chosen we would usually change the offer on UCAS as well as send a separate email directly to the applicant confirming their guaranteed offers.
I hope this helps!
Trinity Laban