This text describes the ABI coreboot presents to downstream users. Such users are payloads and/or operating systems. Therefore, this text serves at what can be relied on for downstream consumption. Anything not explicitly listed as consumable is subject to change without notice.
coreboot passes information to downstream users using coreboot tables. These table definitions can be found in src/include/boot/coreboot_tables.h and payloads/libpayload/include/coreboot_tables.h respectively within coreboot and libpayload. One of the most vital and important pieces of information found within these tables is the memory map of the system indicating available and reserved memory.
In 2009 cbmem was added to coreboot. The "CBMEM high table memory manager" serves a way for coreboot to bookkeep its own internal data. While some of this data may be exposed through the coreboot tables the data structures used to manage the data within the cbmem area is subject to change.
Provided the above, if one needs a piece of data exposed to the OS or payload it should reside within the coreboot tables. If it isn't there then a code change will be required to add it to the coreboot tables.