Your guess is as good as mine
Here's more details on the file format. Note that, since I'm only looking at one instance of the format, and any all of these offsets could be coincidence. It will only be by comparing a large number of these filetypes that we can actually derive information.
Note that it does appear that the data is stored in big-endian format. I think it'd be too much of a coincidence for that to not be the case.
0x08 - points to beginning offset of (compressed or encrypted) data (in this case, 0x2000)
0x10 - points to start of "certificate"? - 2 words before what was previously considered the signature of the file.
0x14 - number of entries that immediately follow. Each entry consists of two uint_32's. The first could potentially identify what the entry means? In either case, the second uint is often a pointer to a specific peice of data contained in the header of the file, and I'll use the first uint to identify each entry. Pretty much all of this is conjecture and unknown
0x000002ff - points to a section of data at 0x70c that has the strings $UPDATES and
MEDIA.. possibly referenced directories?
0x000003ff - points to section starting at 0x730, judging by surrounding entries
to be 36 bytes long
0x000080ff - points to section at 0x754, and is 0x20 bytes in length (judging by next entry)
0x00010001 - doesn't point to an offset in the file. (0x00400000)
0x00010100 - doesn't point to an offset in the file (0x920127f8)
0x00010201 - "" "" (0x92000000)
0x000103ff - points to 0x1b74, ?? contains strings- "xam.xex xboxkrnl.exe",
followed by.. well, something.
0x00018002 - points to 0x0774, which is 8 bytes long (judging by next entry)
0x000183ff - points to 0x077c, string "installupdate.exe"
0x000200ff - points to 0x0794, a list of libraries?
0x00020104 - points to 0x0828, some data. 16 bytes until next table entry
0x00020200 - 0x00010000.
0x00040006 - points to 0x0838, 16 bytes. very similiar to the entry at 0x0828
0x00040404 - 0x0850 - block of nulls until 0x1b74.