From https://infoqubeim.com/drupal5/node/987: 

# 2.2.1   - Backups created on program close (# 2.2.1) use rotating numbers .SNBK0, .SNBK1 and .SNBK2 e.g TestFile.SNBK1 (where .SNBK0 is the current backup, .SNDB1 is 1 day old and .SNDB2 is 2 days old). This is NOT enabled by default.

# 2.2.2 - By default, Auto-Backup (# 2.2.2) is set to every 10 minutes. These backups are currently saved in the same folder as the file itself. After 10 minutes the backup files are replaced if changes have been made to the file. These backups have .SNBKA extensions. 

# 2.2.3  - If you enable the Timestamp option (# 2.2.3) each backup creates a new file, with the date/time in the file name, e.g. TestFile.20090818-1848.SNBKA. Format is always: Filename.yyyymmdd-HhNn.SNDBA (where Hh is hour in 24hr format, and Nn is minutes). Otherwise backups are replaced by the most recent backup. 

# 2.2.4  - Limit the number of backups created:      
This option deletes all but the last timestamped auto-backup of each day that are at least n days old.      
Delete is performed once per day, on the first occasion after midnight.

# 2.2.5  - Select the folder where the backup is to be created. If this is left blank, the backup is created in the same folder as the IQ file. If this is your only backup it is recommended to add a different backup location here, ideally on a separate partition, or even better, on a different hard-drive (click in option # 2.2.5 and browse to new location).

The numbering of these options is no longer the same (are there any changes other than numbering?) - the options now are as follows:

2.4.1 Create backup on program close
2.4.2 Auto-backup every x minutes (0=disabled)
2.4.2.1 Timestamp Auto-Backup
2.4.2.2 Delete Timestamped Auto-Backup files after n days (0=disabled)
2.4.3 Backup folder

The way I'm reading this - 

(1) I'm seeing three types of backup - program close backups, auto-backups, and timestamped auto-backups.

(2) The "program close" backups have extensions .SNBK0, .SNBK1, .SNBK2.  Are the program close backups limited to three?

(3) By the description it seems that they would be made only when the program is closed.  But here is what I am seeing:

The three .SNBK0-2 files all have modification times very close to each other, and on the same day.  The auto-backup is about 10 minutes later, which seems to accord with the specified auto-backup schedule of every 10 minutes.  Do the modification times reflect the time that these program close backups were actually made?  Or maybe for some reason the modification times were altered from the time that these backup files were actually written?  (The creation times are not helpful, they date back to 2020).

(4)  And what happens if the program is left open for days, it would seem that the "program close" backup would never be made during that time?  So that if you closed the program on Day 1, Day 5, and Day 10, that's when the three backups would actually have been made?

(5)  Am I right in thinking that the three "program close" backups are not automatically deleted, but just overwritten on rotating basis?

(6)  Am I right in thinking that if auto-backup is selected, without the timestamp option, there is only one non-timestamp SNBKA auto-backup, and it is not automatically deleted, but just overwritten with each non-timestamp autobackup?

(7)  If the auto-backup is selected and the timestamp option is selected, is there any way to limit the number of timestamped backups other than the 2.2.4 option?  With backups every 10 minutes you could have quite a few by the end of the day. 

(8) Am I right in thinking that even with the 2.2.4 auto-deletes, the last timestamped autobackup for each day remains until it is deleted?

(9) Am I right in thinking that the selected Backup Folder location is used for all three types of backups?