The requirements for use of a salvage drum are explained in the USDOT/PHMSA Hazardous Material Regulations at 49 CFR 173.3(c).  Those for the overpack are explained at §173.25(a).  Both salvage drum and overpack are defined at §171.8.  These are two distinct regulatory terms and yet there is a lot of confusion about which is which and when one is chosen and the other is required.  To answer these questions I refer you to an article I wrote for New Pig:

Salvage Packaging vs. Overpacks:  What’s the difference?

It has the advantage of containing links to more information about each of these packagings from New Pig’s catalog.