What is a structural engineer or designer?
A ‘designer’ does engineering under the supervision of an engineer. The engineer is responsible for the design, and the responsibility given the designer is at his discretion. With experience is structural design, you might be hired in other fields, but you’d have to demonstrate that you can do the required work.
The key difference between structural engineering and mechanical engineering is that a structural engineer works with stationary items, a mechanical engineer normally deals with moving items. The most obvious application of structural engineering is civil engineering. Most people prefer it if their houses don’t move very much. I suppose you could say that architecture is a mixture of structural engineering and art.