TIERS

From MDC

This variable holds the list of tiers to include in the current build. The top-level Makefile.in places some basic stuff here, and then includes an application-specific build.mk file (say, browser/build.mk) to add the appropriate tiers for that application. That file typically includes other build.mk files to add more tiers.

The value of TIERS determines the values of DIRS (in the top-level make, which is different from DIRS in subdirectory Makefiles) and STATIC_DIRS.

How Mozilla's build system works has a more detailed explanation of how tiers work.