I have both .tub and .psptube extensions in my files and both work in X5. I believe .tub was from older versions and .psptube is used in newer versions but they both work. Maybe someone else can explain what the difference is and why it was changed.
They changed the extensions with PSP8. I could be wrong but I think they changed them because other software programs were using some of the same extensions. By adding the "PSP" to them, there would be no mistaking them for anything other than Paint Shop Pro files.
Using Paint Shop Pro versions 5, 6, 7, 8, 9, 12, 14, 16