Doubt on the default NIS /var/yp/Makefile

While reading How to Modify /var/yp/Makefile to Use Specific Databases and inspecting /var/yp/Makefile I have seen the following makefile entries which raised me doubts:

$(DIR)/netid:
$(DIR)/timezone:
$(DIR)/auto_master:
$(DIR)/auto_home:
$(PWDIR)/shadow:
$(DIR)/auth_attr:
$(DIR)/exec_attr:
$(DIR)/prof_attr:
$(DIR)/user_attr:

All the above happen as dependencies to their ".time" counterparts.
But why they are listed as "empty" make targets?
Would they be some sort of "placeholders" for "dynamically generated" sources?

Thanks in advance.