Data, pre- "Generations", famously claims that he cannot feel emotion. However, the exact definition of "emotion" is not made clear in the series, as far as I know. Clearly he feels curious, and has often referred to things as "fascinating". Both of these could be called "emotion".
Is there anything written, either in stories/film or manuals for the writing staff, that clearly define what Data can and cannot do in terms of expressing emotion?
I think what explains your confusion is this. Data doesn't not feel emotion because it's somehow lacking in his design. Data doesn't feel emotion as a safety feature.
Data is an interesting character. For the casual viewer he is just an android with the trope no emotions, and that's good enough. But things get much deeper if you pay attention to the show. Data is the 2.0 model. He has an older brother Lore. Lore was allowed to develop without emotional locks, and he became a psychopath. So for the second model, Dr. Soong added blocks to prevent Data from feeling emotion. This way his positronic brain could mature, and then later he can be given the emotion chip to unlock his emotions.
This is confident with similar ideas to other science fiction like Blade Runner where the replicants are psychopaths because they are made to be adults, but are only only two years old.
So to answer your question, Data can't feel emotions that Dr. Soong considered to be emotions. Just a limit of one self admittingly flawed human being's opinions.
EDIT
Having given this more thought. In the episode where Data creates Lal he says that he can't use contractions. Microsoft Word can figure out contractions. Clearly there are things blocked from his ability to do. Data is unaware of these blocks, so when he creates Lal, she doesn't have them. Because of that she can use contractions and experience emotions.
