Based on the original KNF concept from the Computer Systems Research Group, it dictates a programming style to which contributed code should adhere prior to its inclusion into the codebase.
KNF started out as a codification of how Ken Thompson and Dennis Ritchie formatted the original UNIX C source code.
It describes such things as how to name variables, use indents and the use of ANSI C or K&R C code styles.
Each BSD variant has its own KNF rules, which have evolved over time to differ from each other in small ways.
The correctness of the indentation of a list of source files can be verified by a style checker program written by Bill Shannon.