What is a fixed rate fee?

Could you please elaborate on what exactly a fixed rate fee entails? Is it a type of payment structure where the fee remains constant regardless of the transaction size or duration? And if so, what are the benefits and drawbacks of opting for a fixed rate fee over other fee structures, such as percentage-based fees? Additionally, how does it impact the overall cost-effectiveness for users, especially in the context of cryptocurrency transactions where market volatility can significantly impact the value of assets being exchanged?