If you are a prospective patent practitioner who doesn't qualify for Category A or Category B, Category C is a way to qualify for the "patent bar exam", or officially, the Examination for Registration to Practice in Patent Cases Before the United States Patent and Trademark Office. You can learn more about the patent bar at the Patent Education Series.
According to the USPTO:
"An applicant relying on practical engineering or scientific experience or who does not qualify under Category A or B above may establish the required technical training by demonstrating that he or she has taken and passed the Fundamentals of Engineering (FE) test. The FE test is a test of engineering fundamentals. The FE test is developed and administered by a State Board of Engineering Examiners in each State or comparable jurisdiction. Neither the USPTO nor any other U.S. Government agency administers the test. An applicant desiring to take the FE test should direct inquiries to the Secretaries of the appropriate State Boards. Official results of the FE test must be submitted to establish qualification under this category. An applicant attempting to qualify under Category C must submit an official transcript showing the award of a Bachelors degree."
While this is quoted from the USPTO, it is outdated information. The FE exam is nationally administered by the NCEES.
The FE exam is a comprehensive engineering test that is available in 7 different 'discplines': Chemical, Civil, Electrical and Computer, Environmental, Inudstrial and Systems, Mechanical, and Other disciplines. Choose a discipline based on your educational background and experience, or choose the catch-all Other Disciplines for the most general engineering exam.
PrepFE works differently from other FE preparation services. We are the best option for many reasons. We are technology and analytics driven. Our website is the most modern and advanced while remaining straightforward. Our iOS apps are full-featured and stand well above any competitors' offerings. Our guarantees and customer service are best in industry. All of this comes with the most affordable and transparent pricing.
The content of PrepFE is designed specifically with passing the FE exam in mind. Other resources generally have overly complex, difficult, outdated and sometimes outright irrelevant practice problems and review materials. Instead of being an entire review of an undergrad degree in engineering, our content is focused and streamlined, so you can spend the minimal amount of time preparing, if study time is in an issue for you. To that end, we include many tricky problems, so that you'll be prepared when the NCEES official exam throws the same curveballs at you. And we do also offer challenging problems for our users who want thorough prepration. All in all, if you work through PrepFE and get your scores up, you will have no problem passing the FE exam.