If you're doing no-sparge BIAB, you have things set up correctly. If you're sparging and using differently treated water for the sparge, you need to enter some value in the sparge water section. If you're using the same water and treatments for both, it doesn't matter, as long as the sum of the two is the amount of water you need. Clear as mud, right? Here's my take:
I generally treat mash water only with salts - lowers the amount of salts I need overall. I acidify both mash water and sparge water to ensure mash pH of 5.3-5.6, generally around 5.4, and no tannin extraction. So I enter values for both mash and sparge water and make sure that I'm adding salts only to the mash. If you're not sparging, you'll add salts and acid to the entire volume.