.For questions concerning the promotion and advertising of open projects, and their implications with the general public, society, and respective organizations.
Google defines marketing as
the action or business of promoting and selling products or services, including market research and advertising.
You should use this tag on questions about all aspects of marketing an open or free project: market research, advertising, promoting, selling, etc. General marketing questions are off-topic here, but if you have a problem specific to your open source project then this is the tag you should use.