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Fairly sure with some local rail rovers the bus firms receive a specific payment from rover sales. For the all lines product, only the train companies receive money so not valid. That said, those rail firms do probably own many of the bus companies.
You can, for example, use it on the 100 by D&G Bus for journeys between Stone and Stoke-on-Trent via the intermediate stations Barlaston and Wedgwood, in lieu of West Midlands Trains services calling there.
I'm sure there will be a handful of other examples.
The Cleveland Coast and Whitby Day Ranger allows you to travel on a circular route between Middlesborugh and Whitby using Northern trains and Arriva North East buses X4 and X93, the bus connection being along the coast between Saltburn and Whitby.