Thanks! Domain knowledge - but applying this to a mass market feel a bit mind-bending to me.
Of course, it's still problems and knowledge, and so not qualitatively different from those in a specific niche. I guess the misleading thing is that by the time a mass market is mass, they are well-known (or appear to be).
As an example, Youtube was once small. And while the consumers were perhaps (?) easy to understand, the producers (who created the value), were a much smaller group, a more specific kind of person, using specific new tech (mobile phones) with specific problems. And it only later grew into a mass market (though I guess arguably the producers are still "one kind of customer".)