As explained in this article:

What is the purpose of Garbage Collection?

the Project Cleaner removes projects and tasks that have been completed for the designated periods of time configured in the Projects Cleaner page under

Management > Administration > Garbage Collection > Garbage Collection: Projects Cleaner  

Here is an example of such settings where projects that have been completed or canceled more than 15 days ago are collected and archived by the Projects Cleaner on a schedule set by the WorldServer Administrator. Normally, the Garbage Collection should be run outside of business hours so it is typically set to run at night time.


 

The Projects Cleaner runs these 2 stored procedures when removing a project from the Completed and Canceled Projects list and archiving it:

1. spDeleteProject
2. spdeleteunusedprojectgroup

Note: once completed or cancelled, a project will be present in both the Projects table and the ArchivedProjects table in the Database. After a project has been archived by the Projects Cleaner, it will be removed from the Projects table and will only exist in the ArchivedProjects table.

These articles are relevant:

What is the purpose of Garbage Collection?
WorldServer Transport - what happens if I delete an archived project from the Transport Projects page?
WorldServer - after enabling Maintenance debug logging, which Garbage Collector entries can be found in the log files, especially for Project Cleaner and how to read them?