Additional engine API documentation can be found in the src/doc/design/engine.texinfo file.
This file contains some extra meta-information that is not directly relevant to the API documentation.
API: KVP: Key-Value Pairs
Suppose you have a neat idea for a new feature for the GnuCash engine. Should you create a C structure (a "first class object") and all of the required machinery for it (e.g. an SQL backend), or should you just put all of the data in a suitable KVP (Key Value Pair) frame somewhere?
The answer depends on whether the concept requires extensive pointer chasing between different types of existing C structs, or whether it more naturally can hang with some existing object.
If it seems to be an independent concept, it can still be placed in the KVP tree of the book, which gives it a 'top-level' existence.
If the concept is used only infrequently, then it probably belongs in a KVP tree. If the concept has performance-critical requriements, then it is better to implement it as a C struct, and similarly design an appropriate SQL table around it, so that the database can be queried efficiently and rapidly.