Backend Interface (BKI) files are scripts in a special language that is understood by the PostgreSQL backend when running in the "bootstrap" mode. The bootstrap mode allows system catalogs to be created and filled from scratch, whereas ordinary SQL commands require the catalogs to exist already. BKI files can therefore be used to create the database system in the first place. (And they are probably not useful for anything else.)
initdb uses a BKI file to do part of its job when creating a new database cluster. The input file used by initdb is created as part of building and installing PostgreSQL by a program named genbki.sh, which reads some specially formatted C header files in the src/include/catalog/ directory of the source tree. The created BKI file is called postgres.bki and is normally installed in the share subdirectory of the installation tree.
Related information may be found in the documentation for initdb.
This section describes how the PostgreSQL backend interprets BKI files. This description will be easier to understand if the postgres.bki file is at hand as an example.
BKI input consists of a sequence of commands. Commands are made up of a number of tokens, depending on the syntax of the command. Tokens are usually separated by whitespace, but need not be if there is no ambiguity. There is no special command separator; the next token that syntactically cannot belong to the preceding command starts a new one. (Usually you would put a new command on a new line, for clarity.) Tokens can be certain key words, special characters (parentheses, commas, etc.), numbers, or double-quoted strings. Everything is case sensitive.
Lines starting with a # are ignored.