For at least the past year or so, when a pax does OLCI or uses the United mobile app, if the flight is oversold, there's a screen which displays 3 possible flight alternatives (fairly often not the best ones you'd discuss with an agent) and asks the pax to bid on an amount they'd pay to take each one of those alternatives. I think that is what is being referred to here (the OLCI screen that shows 3 alternatives and asks you to choose your VDB bid in miles or travel voucher dollars for each of the 3 options).

In my experience those specific displayed options have never been the ones I've discussed in person with an agent -- sometimes the inventory is gone by the time we get to the gate, and sometimes there are better alternatives.