Glossary
MRUListEx
A binary registry value listing slot numbers in most-recently-used order, used by BagMRU keys to order their child folders.
MRUListEx is a REG_BINARY value made of 32-bit little-endian slot numbers, most recently used first, terminated by 0xFFFFFFFF. In a BagMRU key it orders the numbered child values.
It is the basis of ShellBags timing: when the parent key was last written, the child at position 0 was the one in use, so that child's "last interacted" time is the parent's LastWrite. Other children get no such time. The same structure appears in RecentDocs and other MRU keys. Details in ShellBags timestamps explained.