The MEMORY_PARTITION_PAGE_COMBINE_INFORMATION structure (formally _MEMORY_PARTITION_PAGE_COMBINE_INFORMATION) both provides input for and receives output from NtManagePartition when given the information class SystemMemoryPartitionCombineMemory (3).

The MEMORY_PARTITION_PAGE_COMBINE_INFORMATION structure is 0x0C or 0x18 bytes in 32-bit and 64-bit Windows, respectively, up to and including Version 2004. Note, however, that the 32-bit form is useless in practice before Version 1703 because the 32-bit NtManagePartition fails trivially (returning STATUS_NOT_SUPPORTED).

Offset (x86) Offset (x64) Definition Remarks
0x00 0x00
PVOID StopHandle;
0x04 0x08
ULONG Flags;
0x08 0x10
ULONG_PTR TotalNumberOfPages;

Among the Flags, only 0x01 is originally valid and only then if operating on the system partition. What flags are valid for later versions is left for another time.