HV probably is "hold value" I think you got it.

It seems to be an offshoot of "MDV", which stands for "Marketing Daily Value". MDV is basically your theo minus comps redeemed -- what they feel they actually make on you average per day.

HV is probably just a raw, per-trip (maybe only last trip) calculation on your theoretical loss per trip.

Why use that instead of ADT?

Because they're giving you a one-use offer of freeplay, and are interested in calculating whether you are likely to lose more than the freeplay value for the trip.

So so it looks like they're giving you roughly 20-25% of your HDV back through freeplay, meaning they're inviting you with the expectation to still get 75-80% of your expected losses.

Never seen that metric before, but it makes sense.