Thanks for referencing official information!

But why can't a National Highway be a primary? The above referenced wiki page (for Vietnam) says that primaries are "National Highways", while trunks are "High-quality National Highways that are not expressways. Usually divided and with partial grade separation." (which is not the case for 40B at the moment).

Do I understand right that 40B is planned to be upgraded to become higher quality till 2030? After that it would make more sense to tag it as trunk indeed.