About Exchange Instant Recovery volume rotation
At the start of a backup, the Snapshot Client is queried to determine how many Instant Recovery snapshots currently exist for each volume. This information is required for the Exchange databases that are selected for backup. If the number of snapshots is currently at the configured maximum level of snapshots, a snapshot is resynced (or snapped backup or deleted). Then a snapshot is available for the upcoming backup attempt.
The algorithm to determine which snapshot volume is resynced considers whether the snapshot was taken as part of a full backup or an incremental backup. The algorithm tries to maintain as many full backups as possible, even if newer incremental backups have to be resynced.