Lilium longiflorum is the wild lily from which the Easter lily of commerce was developed. The commercial forms tend to be very compact in growth and relatively short. The wild forms can be much bigger.
When I first saw 'White Heaven' blooming in my own garden, I assumed that it was a hybrid of some sort. It's bigger than the usual Easter lilies, and it has the tetraploid look. So much work has been done with hybridizing Lilium longiflorum in the recent past that a hybrid origin for this robust plant seemed reasonable.
Earlier this year I checked the lily registry to see what the story was with 'White Heaven'. Evidently it was raised from two forms of Lilium longiflorum, so it's not a hybrid between Lilium longiflorum and another species but rather an intraspecific hybrid.
Here's something else I discovered (and should have known). For show purposes, these plants are to be treated as Division V lilies.In all of the shows I've attended in the past, Lilium longiflorum has been placed among the species.