Suppose you have build items A, B, and C, and suppose that B doesn't actually require C to build, but anyone who needs B also needs C. In this case, B should declare a dependency on C, or B and C should be combined. In other words, a build item should depend on all build items that will be needed if you use it.

Consider a concrete example. Suppose our three build items are main, lib-headers, and lib-src. Suppose lib-headers doesn't have an Abuild.mk and doesn't actually build anything. Instead, it just has an Abuild.interface that adds its directory to your INCLUDES variable. Suppose lib-src builds a library and has an Abuild.interface that adds the library directory to LIBDIRS and the library to LIBS. If main uses the library built by lib-src but declares a dependency on lib-headers, then it will be able to compile but not link. In order to link, it requires a dependency on lib-src. This means that anyone that depends on lib-headers must also depend on lib-src. Rather than having this situation, make lib-src's Abuild.interface append to INCLUDES and just eliminate the lib-headers build item entirely. It is still okay to have the headers in a separate directory; just don't place an Abuild.conf in that directory.