Generally our perception and reaction to external stimuli are at the conscious level, when we are fully aware.
Subliminal messages are crafted and delivered in such a way that both the perception and reaction to these happen at the subconscious level. The audience cannot be aware of these even if they try to, because whatever happens is below the threshold of our conscious awareness.
Supraliminal messages on the other hand are designed so that the perception happens at both the conscious and subconscious level. The audience is conscious of the stimuli, but generally unaware of the strategy applied. However, the intended reaction happens at the subconscious level. Highly alert people can also detect the strategy, in which case, the intended strategy may fail.