It is valid as it gives you a Day Travelcard.
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It may say it is giving you a day travelcard, but I'm not conviced it is. There are two zonal destination descriptions and they allow very different things:
U1256 is a single or return journey from the last National Rail station to any Underground or DLR station in the zones covered. It is not valid for multiple journeys, nor on any National Rail services once the Underground/DLR portion has commenced.
Z1256 is a day travelcard valid on all services in the zones as many times as you like.
Note, there are never more than 4 numbers present, the middle one(s) are left off if 5 or 6 zones are required.