Sorry to perhaps sound clever about this but I've always understood MBR to be Master Boot Record, a 512-byte boot sector that is the first sector of a data storage device such as a hard disk. It can be used for (as the wiki says

):
# Holding a disk's primary partition table
# Bootstrapping operating systems, after the computer's BIOS passes execution to machine code instructions contained within the MBR
# Uniquely identifying individual disk media, with a 32-bit disk signature; even though it may never be used by the machine the disk is running on
If this boot sector gets damaged, you can recreate it by using FDISK /MBR
Now I think I know what Leven means, but in answer to the question.... if you do a
system restore it restores every file. If you do an
upgrade to a later kernel (as v70 is) you just replace files that have been updated, not replace all of the files on the computer.
Now.... an individual file may have been updated because of a bug in it, a better way of doing something in the file, or recoding so it takes less memory.
Hope this helps.