To me, it is first defined by what it isn’t. It isn’t an editor of any type. Spelling, punctuation, grammar, and word use are not requested, except to clarify what the reader Thinks what is meant by the writer.
- The beta reader reads for enjoyment, stopping for non-enjoyment tasks such as noting:
- Confusing scenes
- Timeline/plot/location discrepancies.
- Confusing characters,
- Confusing character arc, good->bad, bad->good, silly->deliberate, etc.
- Confusing or unrealistic dialog or relationships
- Questions about facts. As an example, how did the four-foot-eleven woman accurately use the 10 mm semi-automatic that’s larger than her hand?
- Questions about tropes and memes. Especially valuable if you are deep into a genre, and golden if you aren’t. The latter helps rewriting decisions to add material to hopefully add to the reader base.
- The consistency of Themes, or even if there is one.