And, for emphasis, miles + cash tickets book into award inventory (basically why GUCs cannot be used) and are based on the price in miles of an award ticket.

In contrast, pay with miles tickets book into regular inventory (like tickets paid entirely with money), but you decide how many miles to use to reduce the price of the ticket, at a rate of approximately one cent per mile with some rounding. They earn FF credit unless you pay for the entire fare with miles.

Sometimes pay with miles can be a better deal than an award ticket on the same flights and conversely sometimes cash and miles can be better than buying the same ticket for cash, although in principle dynamic award ticket pricing should mean that one shouldn't find big differences.