Recently there’s been a lot of checkins to the EDK-II trunk. Most are new libraries for UEFI 2.5 support, but some are design-time improvements to the EDK-II toolchain. One recent change to the BaseTools package is to implement BUILDRULEORDER support for tools_def.  This adds increased flexibility for compilers and related developer tools, which EDK-II’s Build command calls behinds the scenes. Quoting it’s comments:

This feature allows the toolchain to choose a preference for source file extensions in tools_def.txt. The first extension is given the highest priority. Here is an example usage for tools_def.txt:

*_*_*_*_BUILDRULEORDER   = nasm Nasm NASM asm Asm ASM S s
*_XCODE5_*_*_BUILDRULEORDER   = S s nasm Nasm NASM

Now, if a .inf lists these sources: 1.nasm, 1.asm and 1.S
All toolchains, except XCODE5 will use the 1.nasm file. The XCODE5 toolchain will use the 1.S file.

Note that the build_rule.txt file also impacts the decision, because, for instance there is no build rule for .asm files on GCC toolchains.

For more information, view recent archives of the EDK2-devel mailing list, such as:
http://comments.gmane.org/gmane.comp.bios.tianocore.devel/14760