[U-Boot] in Kbuild, what is the difference between "extra-" and "obj-"?

admittedly, this is more a kernel Kbuild question, but in common/Makefile, i see the snippet:

extra-$(CONFIG_ENV_IS_EMBEDDED) += env_embedded.o obj-$(CONFIG_ENV_IS_IN_EEPROM) += env_embedded.o extra-$(CONFIG_ENV_IS_IN_FLASH) += env_embedded.o

what is the functional difference between using "extra-" and "obj-" in the above? those three lines look ... inconsistent?

anyway, what is going on there? thanks.

rday