In my experience it applies to coding when you have any reliance on third party libraries or services and don't have an extensive amount of actual real world experience with that technology already.
If you have unknowns, then your planning process starts with, "let's figure out how to use this new technology." And that process can involve a bunch of prototyping.
Having to make a choice between "make a design document" or "do prototyping" is a false dichotomy. They're complimentary approaches.
Having to make a choice between "make a design document" or "do prototyping" is a false dichotomy. They're complimentary approaches.