I am creating a model that needs to ensure that all input datasets end up in the same coordinate system (among other things). I was originally going to batch project all of the datasets whether they need to be projected or not, but then there's the issue of assigning the correct geographic transformation. Then I remembered that all data that is input into a feature dataset has to be the same coordinate system, so I decided to put all the data into one.
My question is: When a feature class is imported into a feature dataset that is of a different coordinate system, does it automatically project the data? What happens?
I tested this myself on roads data and it seemed to project the data, as they lined up nicely with my other data. The thing is, no transformation was needed (and a transformation method was needed had I projected it), so it seems fishy that the data is just automatically projected. Maybe it makes a best guess?