Mint in sealed box (commonly abbreviated as "MISB"; variations include "MIB"/"Mint in box", "MOSC"/"mint on sealed card", and "MOC"/"Mint on card") refers to the practice of keeping toys, including those of the Transformers brand in their packaging, even after purchasing them. Such specimens are valued more than "loose" toys—toys that have been removed from the packaging. In addition to the perceived "germ free" nature of such a toy, the owner can be assured that the item is almost certainly complete and intact. The chances of a still-sealed action figure being broken or missing accessories is extremely unlikely; such cases are almost always due to either accidents at the factory (toys being packaged with wrong/no accessories), or the toys being made of materials that decompose relatively quickly regardless of how they are or aren't packaged (such as yellowing, Gold Plastic Syndrome, and rubber rot).
Some fans take umbrage with the practice of collecting sealed toys, arguing that the entire point of collecting action figures is to free them from their cardboard and plastic prisons and play with them, while fans of sealed figures might compare the process to collecting fine art or artifacts - they enjoy the total aesthetic of toy and package, looking much as it did the day it was placed on store shelves long ago. A handful of collectors buy sealed toys for the distinct pleasure of being the ones to open them, which must undoubtedly frustrate die-hard sealed collectors as MISB toys are a finite resource.
Officially, the AFA will only accept MISB and MOSC toys for its grading service, although mistakes sometimes sneak through the cracks.