I haven't found it spelled out explicitly in any documentation, but this is my take on precedence among the various jam files. Please correct me if this is wrong, or too simplistic.

Jambase: Compiled into jam executable, not referenced thereafter.

Jamrules: (Optionally) Exists at the root of multiple-compile-unit source trees. Provides common rules within the tree. Overrides Jambase.

Jamfile: Exists within a specific compilation-unit directory. Provides specific compilation rules for the sources within the directory. Overrides Jamrules and Jambase.

Correct? Close? Laughably naive?

thanks, Dave