Moisture Vapor Barrier (MVB) shielding bags, better known as Moister Barrier Bag (MBB), are a special subset of metallized bags as they also have the property of EMI-RFI-ESD shielding. This is mainly accomplished by using a much thicker metal layer (about 10x thicker than standard metallized bags), which inhibits the moisture vapor transmission rate (MVTR) by a factor of over 20 times more compared to ordinary shielding bags.
In general, if you are storing ESDS devices for prolonged periods of time (6 months or more) or if the devices are sensitive to corrosion, than you should package these materials in either an MVB (Moisture Vapor Barrier) bag with a desiccant pack to absorb any moisture that was sealed in or a metal-in shielding bag with a desiccant pack. The difference between an MVB film and a metal-in film is about 1 magnitude in the moisture vapor transmission rate (grams of water/100 in2/24 hours @ 100 oF). Moisture sensitive ESDS devices should only be packaged in an MVB bag.
In both cases, the bags MUST be sealed to properly keep moisture out.
For all other ESDS (ElectroStatic Discharge Sensitive) devices [that are not moisture sensitive], a metal-in, metal-out or a clear dissipative bag can be used. Heat sealing is much preferred, however, you can often fold over the top of the bag and close with an ESDS sticker for adequate protection.
ESD bags should be inspected before reuse by an internal statistical sampling plan as they typically have a finite reuse life.