I think the confusion is due to how the "View deleted items" view option works, which is different than the "Keep deleted items" archive settings. I'll try to clarify using the following example:
(file F created)
Layer 1: captured a folder containing file F.
(file F deleted)
Later 2: recaptured the same folder, this time there is no file F.
The archive now has two layers. Layer 1 contains file F, layer 2 doesn't. File F is now a "deleted item".
Opening and viewing this archive (with all layers shown) will not display file F, because in layer 2 file had been deleted. If you then turn on the "Show deleted items" view option, file F will appear as a deleted item (hash background) because it previously existed in an earlier layer (1) but doesn't exists in the lastest layer (2).
At this point the file still exists as a regular file in layer 1.
Now, merge the two layers with the "Keep deleted items" setting turned off. When layers are merged only the most recent version of each item retained and files that have been deleted are removed from the archive. After the merge there is only one layer, and there is no file F. If you turn on the "Show deleted items" view option, you still won't see file F because it isn't there.
If the "Keep deleted items" setting had, instead, been set prior two merging the two layers then the file wouldn't have been removed from the archive. Assuming the latest version was within the "Keep" time period, instead of expunging the file from the archive QRecall would preserve the latest version in a special file record marked as having been deleted.
In the second scenario, if you once again open the archive you don't see file F. Even though the file is stored in the layer, it isn't shown because it's a "deleted" file. If you then turn on the "Show deleted items" view option, the file once again appears as a deleted item in the browser.
So the "Show deleted items" view feature will show items captured in earlier layers that have subsequently been deleted. It will also show the special "deleted" items preserved in a layer via the "Keep deleted items" setting.
The deleted items perserved in a merged layer are finally deleted for good under two circumstances. If the layer is merged again, and this time the deleted items it contains are older than their keep-until date, the items are deleted just as they would if the "Keep deleted items until" setting was off. The compact action also sweeps merged layers looking for preserved items that are now too old to keep.
Now, back to your questions:
That is a "deleted file" in my understanding and it is not created by any merging actions but simply derived from a change in filesystem.
Correct.
According to my understanding, such a file (after a grace period set in archive preference, in my case 200 months) will be completely erased in any case by a compacting action/command even if I don't merge any of the archive layers.
Based on your reply it seems that, if I don't merge layers N and N+1, the files that I deleted in finder between the captures remains forever in the archive (as they belong to layer N that is still there).
That's correct. A "deleted item" mean "doesn't exist in a recent layer, but may still exist in a previous layer."
The confusion rises from the term deleted used in two different meaning: what will be erased from a compacting action are the "unallocated" items (the layer which they belong to doesn't exist any longer) while "deleted" items from file system are preserved.
It shouldn't cause any confusion if you remember that the "Keep deleted items for at least" setting is not a "keep deleted items exactly XXX days" setting, it's a "Keep delete items
for at least XX days". It doesn't set a fixed window in which all deleted items are removed from the archive. It simply creates a grace period in which items that would have
normally been deleted by a merge action are, instead, not deleted. Once the item is older than that grace period, the normal item removal rules apply.
May I suggest to use two different terms for the two file states? The Help is not so clear as well about "deleted items".
I don't think there needs to be two terms, because it shouldn't matter. From the user's perspective there are only "deleted" items. How long they exist in the archive is a combination of how the layers are merged and the "keep deleted items ..." setting.
If you want to see the difference in the browser, there's an advanced setting (QRBrowserDistinguishDeletedItems) that will reveal the difference between a deleted item captured in an earlier layer and one being preserved in a merged layer. See the
1.2.0b46 release notes for an explanation and a screenshot showing the difference.
You can also disable the compact's deletion of "deleted" items by changing the QRCompactErasesExpiredDeletedItems advanced setting to false.