Since there are the different sections of the chest, Infected Files, User Files and System Files (excluding the collation section of All Chest Files). So you have to exercise care in what you delete.
The Infected Files section is self explanitary, infected files that avast has detected and you elected to send to the chest.
The User Files section is if you feel a file is suspicious but not detected by avast, this is where you can add it, so it can do no harm whilst you investigate further.
System Files are back-up copies of important system files (copied by avast), in case the originals get infected.
So only those in the Infected or User Files section should be deleted with this provision. There is no rush to delete anything from the chest, a protected area where it can do no harm. Anything that you send to the chest you should leave there for a few weeks. If after that time you have suffered no adverse effects from moving these to the chest, scan them again (inside the chest) and if they are still detected as viruses, delete them.