I understand this as follows: Legal: doesn’t break the rules of chess. For example: no pawns on the eighth rank or in check when it’s not their turn. Reachabl

I understand this as follows: Legal: doesn’t break the rules of chess. For example: no pawns on the eighth rank or in check when it’s not their turn. Reachable: there is a series of moves that lead to this position from the standard starting position.