Corporal Vs Specialist

Haven’t gone to basic yet and can’t get a clear answer from RSP but from my understanding a corporal is someone who is going to be promoted to sargeant but hasn’t gone to BLC yet. Is this all it is? It seems that some people think corporals are a good thing and a sign you’re on the right track to become an NCO but others seem to think being laterally promoted to corporal is a bad thing that means you weren’t good enough to be a sargeant. Which is true? Are corporals rare?