The work content of field service engineers
"Engineers who provide services directly in the field" are called field engineers and service engineers.
Engineers are usually behind the scenes, and their role is to build / maintain / operate the company's systems. Even in a system production company, most engineers do not show up, and their main job is to build the system as requested by the customer for the projects won by the sales staff.
The decisive difference between such engineers and field engineers / service engineers is that they "go to customers and provide services." To give a general example, field engineers and service engineers perform replacement work due to aging hardware.
They face customers directly and work on hardware / software on the fly. However, unlike sales, we do not proceed with business negotiations, so we are different from system engineers and sales engineers.