The game's sources were written against the Xbox Development Kit (XDK): they include xtl.h and a few other SDK headers, and use its Win32 subset, Direct3D 8, DirectSound, Xbox networking and debug APIs. The native ports (Linux, Windows, Android) implement those APIs themselves (port/linux/src), so from the SDK they only need the declarations: the types, structure layouts, constants and function prototypes. The SDK cannot be redistributed, so these headers stand in for it. They were written without the SDK's headers, from the sources below, and the native builds need no part of the XDK.

What is here

Only what the game and the platform layer use:

File	Contents	Written from
`xdk_pdb.h`	types, structures, enumerations and function prototypes	the January build's debug information (generated, see below)
`xdk_win32.h`	Win32 constants and macros	public documentation, public-domain mingw-w64 headers
`xdk_winsock.h`	Winsock and Xbox network constants and macros	public documentation, public-domain mingw-w64 headers, the January build
`xdk_xbox.h`	Xbox system and controller constants	the January build, public Xbox documentation
`xdk_d3d8.h`	Direct3D 8 (the Xbox's) constants, macros and the C interface wrappers	the January build, public Xbox and DirectX documentation
`xdk_dsound.h`	DirectSound (the Xbox's) constants and wrappers	the January build, public documentation
`xdk_xbdm.h`, `xdk_xkbd.h`, `xdk_d3d8perf.h`	debug monitor, debug keyboard, Direct3D performance counters	the January build, the platform layer's implementation

and, under the names the sources include, headers that bring them in as the SDK's did: xtl.h (everything; NOD3D and NODSOUND leave out Direct3D and DirectSound), windef.h, winbase.h, xbox.h (which also gives the debug keyboard to a unit that defines DEBUG_KEYBOARD), xkbd.h, xbdm.h and d3d8perf.h. Each hand-written header says, per group of declarations, where it came from.

Where they came from

The January build's debug information. cachebeta.pdb, the debug information of the build the decompilation reproduces, records every type, structure layout, enumeration and library function prototype the game was compiled with. tools/xdk_headers.py (with tools/pdb200_types.py, a reader for its PDB 2.00 format) writes the ones named in pdb_names.txt, and what they depend on, into xdk_pdb.h, then checks every structure's size and member offsets against the PDB for each port's target. To add a name: add it to pdb_names.txt and rerun python tools/xdk_headers.py --pdb path/to/cachebeta.pdb.
The January build's code. Macros (#define constants) leave no trace in debug information, but their values do in the compiled game: where the game's source uses one, the original machine code shows its value, and most of source/ compiles to exactly that code.
Public documentation and headers. Microsoft's public Win32, Winsock and DirectX documentation; the public-domain headers of mingw-w64; the Xbox Dev Wiki and nxdk for Xbox-specific facts.
Our own code. The C interface wrappers (IDirect3DDevice8_... and the like) and anything else the SDK defined inline are written here, forwarding to the functions the platform layer implements.

The hand-written headers were written by agents given only these sources and the list of names our code uses, never the SDK's headers or anything derived from them; the generator reads only the PDB. The few additions made while integrating them (APIENTRY, <ctype.h>, the C++ linkage of the Direct3D tables) are public Win32 facts or C plumbing.

How they are checked

tools/xdk_headers.py checks each generated structure's size and member offsets against the PDB, for Linux, Windows and Android.
The game's units were compiled with the original compiler and these headers, and compared with the January build: every function that matched the original with the SDK's headers calls the same functions with the same constants and data with these. Where the code differs, it is only in form: the inline functions here are written differently from the SDK's (a switch where it had a chain of comparisons, a loop where it had sixteen stores).
The native builds compile for all three targets, and the game runs.

Some values leave no trace in the January build, because nothing in the game tests them; those were chosen to fit the platform layer, and say so where they are defined: XBDM_NOERR, the individual DSMIXBIN_* bits (the game only uses their combinations, which the build fixes), which of the XNET_GET_XNADDR_* bits means Ethernet and which DHCP, XMO_STREAMF_*, DSSTREAMPAUSE_*, and the field names of the debug monitor's module records beyond those the game reads.

Changing them

They are ordinary headers, maintained by hand apart from xdk_pdb.h. When code starts using an SDK name that is not here: if the PDB describes it, add it to pdb_names.txt and regenerate xdk_pdb.h; otherwise add it to the topic's header with a comment saying where its value comes from (the January build, public documentation, or the platform layer's own choice). Never copy from the SDK's headers.