A substantive office action is one in which the USPTO's examiner raises concerns that go beyond mere formalities with the application. It indicates that there are significant concerns that need to be addressed before your trademark can be approved for registration. The issues are usually more complex, and the responses require more time and effort to prepare.
The most common reasons for issuing a substantive office action are based either on Section 2(d) of the Trademark Act, which concerns the likelihood of confusion with other marks, or Section 2(e), which concerns the descriptiveness or genericness of the mark.
Responding to a substantive trademark office action typically requires providing legal arguments and evidence. It may also involve amending the application. It's advisable to consult with a qualified trademark attorney to effectively respond to a substantive office action and increase your chances of successfully registering your trademark.