The ultimate truth is that while there are "industry standards" as to when to use PS or AI, you can use either for just about anything as long as you know the limitations and advantages of each—as well as how to get the best results for your particular need for each. For instance, you can save a Photoshop file as a PSD to retain the vector output of text and shapes, but any raster image in the file will still have to conform to resolution restrictions.