@William Hochstedler Here's the e-mail I received from a Burt Harvey
The undivided interest only applies to improved properties that are occupied. Unimproved land and abandoned properties are usually sold to the highest bidder for 100% interest. Yes a tax deed is the means of conveyance. It will grant ownership, but usually the new property owner will need to take additional legal steps, such as a quiet title action, before the property could be used or sold by the new owner. It is my understanding that title companies will not insure a property transaction for a tax sale property unless there has been some other sort of legal action taken, like a quiet title, to secure the property after the sale.
Also, yes, your assumption is correct in an undivided interest situation. The tax sale bid winner would share the ownership of the property with the original owner. The following link to Utah State Law governs how that relationship is handled.
https://le.utah.gov/xcode/Title59/Chapter2/59-2-S1351.7.html?v=C59-2-S1351.7_1800010118000101
Burt Harvey