applicationCompiler fileset.resources vs fileset.depfun

Anyone know how the applicationCompiler decides when a source file is classed as a resource vs a regular dependency? I have a set of similar files - all contain constants used by various source files and a few static methods? Some of these are under <fileset.resources> and some under <fileset.depfun> in the PRJ file with no obvious rule for which goes where that I can figure out?