A lot of aftermarket
I hate to answer a question with a question, but why are you asking an Internet forum about the meaning of their verbiage? They are the only ones that can provide the answer.
For the ND Miata a lot of aftermarket transmission fluids are recommended. Someone at Miata.net quoted this from Mazda site. The Operator’s Manual calls for service, either severe or normal period. The OEM specs calls for Original Mazda Long Life 75w-90 IS and then says if that fluid is not available then any GL-4 75w-90 will do but may cause hard shifting in cold weather.
Everything points to You can change the fluid at the mileage/time per schedule. Then today I see this from Mazda. So I looked around on the internet and see the “Lifetime Transmission fluid” verbiage on quite a lot of car manufacturer transmission. Wondered if this is a work around legal thing.
As far as sealed transmission, from what others have said if there is no dipstick it is that definition. But what does anyone know?