Perhaps this would be a better explanation.
A full space-time continuum is a 4-dimensional construct of technically infinite 3-D size. All matter and energy within the 3-D confines of that 4-D structure make up what we know as the universe.
3-A is Universe level. This is used for characters who can destroy the observable universe/a finite universe and anything above that.
High 3-A is High Universe level. This is used for a character who has infinite 3-D power, allowing them to destroy an infinite universe/a character with 4-D scale power below the level of a full space-time continuum. As our physical universe is possibly infinite, this is used as the highest bounds of 3-A.
Then, there's Low 2-C, which is Universe level+. This is used for characters who can destroy the entirety of a space-time continuum. As I said, this is equivalent to destroying a universe at every possible individual point in its existence, past, present, and future. A being who is itself 4-D would also exist outside of our conventional view of time, allowing them to experience large portions of it at once and often travel "forward" and "backward" along a timeline, at will.
As of now, Beerus only qualifies for the first category, which as I said before, stretches from destroying the observable/a finite universe once to any number less than infinite times.