As noted above, you can view all fare class availability by enabling Expert Mode. From there, you'd initiate a revenue search instead of selecting "Book with miles" on United's website.

Your United fare class will impact your ability to get upgrades and earn elite qualifying miles and segments. Many or all of the products on this page are from ...

Again, the IN fare is only available to Premier ... business-class saver awards if there is saver availability in economy class. According to United's air award rules, you can call and book ...