Fdbbackup version and snapshot and their relationship to restore

I have some questions regarding backup and restore. I run continuous backup in my cluster and it is setup to create a new snapshot every 10 days and incremental in between. Also, I run fdbbackup expire to expire oldest snapshots and only keep 1 complete snapshot.
Here are my questions:

  1. When fdbbackup expire run, what exactly are removed? I know the older snapshot (over 10 days old) will be removed, Would it also reduce the incremental logs that are older than that snapshot version?

  2. When I run fdbbackup describe, here is what I see:
    fdbbackup describe -d file:///var/backup_volume/backup-2023-07-25-19-46-59.778157/
    URL: file:///var/backup_volume/backup-2023-07-25-19-46-59.778157/
    Restorable: true
    Snapshot: startVersion=331384776002 (maxLogEnd -17.99 days) endVersion=864058615363 (maxLogEnd -11.82 days) totalBytes=615751308 restorable=false expiredPct=100.00
    Snapshot: startVersion=867540128148 (maxLogEnd -11.78 days) endVersion=1728013314717 (maxLogEnd -1.82 days) totalBytes=652005925 restorable=true expiredPct=0.00
    SnapshotBytes: 1267757233
    ExpiredEndVersion: 864047446682 (maxLogEnd -11.82 days)
    UnreliableEndVersion: 864047446682 (maxLogEnd -11.82 days)
    MinLogBeginVersion: 864047446682 (maxLogEnd -11.82 days)
    ContiguousLogEndVersion: 1885599736718 (maxLogEnd -0.00 days)
    MaxLogEndVersion: 1885599736718 (maxLogEnd -0.00 days)
    MinRestorableVersion: 1728013314717 (maxLogEnd -1.82 days)
    MaxRestorableVersion: 1885599736717 (maxLogEnd -0.00 days)

The first snapshot with startVersion=331384776002, it has a restoreable=false, is that a incremental?
The second snapshot with startVersion=867540128148, that’s the most latest complete snapshot with restorable =true, right?
So, if I need to restore to the latest, it will take the snapshot of the second one (startversion=867540128148) + any incremental logs? And this is a full set from beginning up to the MaxResorableVersion?

  1. What exactly is version means here? Is it a state of time of the DB at a particular timepoints? ie, if restore to a particular version, I will get all the data up to that timepoints inside the DB no matter how old the data would have been? Thus, still a complete set up to that time point, right?

  2. Based on 3, the MinRestoreableVersion is also a point in time where the DB is at, and when restoring, it is just incremental to the MaxRestoreableVersion to get it up to date to that time point, is that correct?

  3. What is expirePCt means? The first snapshot (startversion=3313847776002) has a 100 expirePct and its latest version is smaller then the MinRestorableVersion, would it ever get purged when running fdbbackup expire command with a version that’s based on the current MaxRestorableVersion?

Thanks for explaining.