$(modules:.ko:.o) Advanced Features for Reference to Variables

I can't understand Grammar of makefile_modpost: $(if $(KBUILD_MODPOST_NOFINAL), $(modules:.ko:.o),$(modules)),when i browse kernel in scripts/Makefile.modpost at https://github.com/torvalds/linux/commit/ea837f1c050344c3f884531a195c6e339b1a54e8#diff-742ab4bcbdb44545def9a3982e0ac3b0.what does it mean?$(modules:.ko:.o)Isn't Advanced Features for Reference to Variables?like Substitution References?bar := $(foo:.o=.c)why don't use $(modules:.ko=.o) instead of $(modules:.ko:.o)?where can i find the grammer to explain $(modules:.ko=.o)???