The term "ftm-sub" in the context of a porn video tag refers to a specific niche within transgender and alternative adult content. It stands for Female-to-Male (FTM) submissive.
FTM individuals are those who were assigned female at birth but identify as male or in some cases, nonbinary. The term "sub" denotes submission, which refers to a role within BDSM and kink play where one person willingly relinquishes control to another.
Therefore, this tag represents adult content featuring FTM individuals engaging in submissive roles, such as receiving dominance or control from others (usually female or nonbinary partners). This genre of porn caters to the preferences of viewers who are specifically interested in seeing FTM-identified individuals exploring submissive fantasies and kinks.