What mkis said.
There are 4 sections of the chest. (AKA quarantine.) All areas are secure; nothing can get in or out without specific user input via the Avast program.
The 4 sections are "infected", "user", "system", and "all".
All you really have to be concerned with is "infected". Files that are in that section have been flagged as malware, and they usually are. Occasionally there will be a false positive in there. Periodic re-scanning from inside the chest is recommended. If the file still scans bad after, say, 2 or three weeks, and the computer seems to be performing normally without that file, it can safely be deleted. (Note, there is no urgency to do this.)
File in the "system" area contain file information about some selected system files, in the event the actual file becomes unisable due to a virus, a repair may be possible based on the stored information. These files are placed in the chest following a "VRDB" scan, and are updated from time to time..I think the period is every 21 days.
Don't touch them.
The "user" area is an area you can add files to, for protection. (Either the file, or your system if you suspect the file may be bad.) If you suspect a bad file that isn't detected by Avast, it can be moved here, and emailed to Alwil for further analysis. Not a bad system at all.
The "all" section allows viewing of all categories. This is what has caused doubt for you. Really, only be concerned with the "infected" section.
Opening the chest and clicking "Help" (top right) offers excellent advice. Some of which I've referred to in forming this reply. Not a bad idea, if you get my drift. (Not that I mind helping here at all.
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