See the Linker and Utilities Guide for your toolchain for more information.BUILD_LIB
(Project Properties window)
*.prj\BUILD_LIB
The BUILD_LIB group is displayed when you create a Library project. It contains settings that combine the object files created by the COMPILE, ASSEMBLE, and CUSTOM groups into a library. This group invokes the librarian after all other files have been checked for rebuild, so you can only have one BUILD_LIB group for a project.
The BUILD_LIB group contains the project settings for the librarian or archiver. For ARM tools, this is normally the program armar.
See the Linker and Utilities Guide for your toolchain for more information.