What is a fixed service fee?

Could you please explain to me what exactly a fixed service fee entails? I'm interested in understanding the specifics of this term in the realm of cryptocurrency and finance. Is it a one-time charge that remains constant regardless of the transaction size or the volume of services provided? Or does it vary in some way? Additionally, how does it compare to other types of fees, such as percentage-based fees, in terms of its impact on the overall cost of transactions and services? Your insights would be greatly appreciated.