Research engineers turn theory, pseudocode, or smaller proof of concepts into a more fleshed out implementation. Once a research project exceeds a few thousand lines of code it becomes useful to have dedicated engineers doing architectural design, owning unit testing / backtesting frameworks, code quality control, etc.
Source: was a research engineer at Intel Labs several years ago.
While this is true, it is also sometimes merely based on your degree. I was a "Research Engineer" doing the same work as "Research Scientists" because my degree was in "Computer Engineering" not "Computer Science."
