As a professional who has worked within the Film and TV industry for years I can say that all 'mastering' for TV deliverables is done between 64-940 (a 10 bit para 8 bit 16-235), with no over or under range data - what we call Super Black and Super white, but what I see referred to in AV forums as BTB and WTW.
However, the final mastering of the DVD/Bluray discs can often introduce excursions above and below 64-940, in part due to compression artefacts within the encoding chain.
But, such excursions do not normally contain valid image information.
However, the actual specification is a 'guide' rather than fixed rules, and so long as the excursions are not beyond 4-1019 all is considered as valid.
The chances of ever finding valid image data beyond 64-940 is as close to zero as makes no difference.
It is all rather confusion - even for Film and TV professionals!
As has been discussed elsewhere on this forum recently, there is no 'broadcast' material that includes valid image detail below 16, or above 235.
It is a 'legal' requirement that all material supplied to all broadcasters, and to all disc mastering houses, is within the broadcast legal range. If it is not, it will be rejected.
There is a whole sub-industry within the film and TV world dedicated to verification of such legal levels.
Where there are excursions above/below the legal range thwy are (usually) compression noise/artefacts, and not valid image detail.
There are some reference discs that have valid detail - and I guess a disc or two do gets made incorrectly now and again - but that's about it.
The rules for TV broadcast imagery, including DVD and Blue-Ray discs is simple.
No valid image detail outside 16-235.
Anything outside that range is illegal, and if a post-facility attempted to deliver material that had illegal content it would be rejected.
It really is that simple.
