NTOSDEF_X.H

The public symbol file NTKRPAMP.PDB for the original release of Windows 10 tells that the kernel is built with the NTOSDEF_X.H header at

d:\th\minkernel\ntos\inc

and draws from it the following type definitions:

Line Number	Type	WDM.H	NTIFS.H	NTOSP.H
252	struct _GENERAL_LOOKASIDE	15300		752
272	struct _GENERAL_LOOKASIDE_POOL	15320		772
300	enum _MODE	15348		800
383	struct _KAPC	15366		883
519	struct _KDPC	15486		1019
587	struct _KREQUEST_PACKET			1087
596	struct _REQUEST_MAILBOX			1096
714	struct _MDL	15612		1214
807	struct _SECURITY_CLIENT_CONTEXT		12932	1305
957	struct _DISPATCHER_HEADER	15729		1416
1279	struct _KEVENT	15873		1561
1291	struct _KGATE	15885		1573
1301	struct _KTIMER	15895		1583
1321	enum _LOCK_OPERATION	15915		1603
1330	struct _FAST_MUTEX	15924		1612
1373	struct _KTIMER_TABLE_ENTRY
1381	struct _KTIMER_TABLE
1388	struct _KTIMER_EXPIRATION_TRACE

The header NTOSDEF_X.H is not known in any Device Driver Kit (DDK) or Windows Driver Kit (WDK).