While enabling cloud tiering, date policy and volume free space policy are 2 different policies that can be used to update Azure File Sync the moment cool files should be tiered.
In volume free space policy, the tiering of cool files is done when a particular space amount is consumed on your system. While in date policy, tiering of cool files is done if they have not been accessed (written or read) for 'y' number of days.
Reference:
https://docs.microsoft.com/en-us/azure/storage/file-sync/file-sync-cloud-tiering-overview