The first batch means that you are eligible to apply for key schools.

2A is the second batch of A-lines, and this A means that you are eligible to apply for an ordinary undergraduate school.

2B is the second batch of B-lines, and this B is a B-line (relatively expensive) that is eligible to apply for ordinary undergraduate schools.

3A is the third batch of A-lines, and special A refers to a college (mostly public schools).

3B is the third batch of B-line, and special B refers to applying for B-type junior college (mostly private schools, the price is more expensive than A-type schools).

1A 1B makes no difference, because 1 is a key undergraduate course. ...

This is an ordinary undergraduate course.

Just a different name.