The most important requirement for Form I-129F is that the fiance gets married within 90 days of arriving to the United States. It is the sole purpose of this visa.
Although the K-1 visa is a non immigrant visa, it is unlike other non immigrant visas in that it does not require the applicant to show that they will return to their home country after their brief stay in the United States. Rather, they will be expected to file for a Green Card shortly after arriving in the U.S.
This makes this visa “dual-intent”, meaning that they come to the U.S. as a non immigrant, but then become a permanent resident (Green Card holder) during their stay.
Other requirements may include a visa interview, biometrics appointment and various supporting documentation to be included in one’s application.