Oh yeah, the instruction is so confusing... But what they mean about Police "Clearances"/certificates is nbi clearance and police clearance. If for say someone passed his/her application w/ only nbi clearance on it later in the process they will asked for police clearance which will add another time for processing the apps as they will have to check that again.
That is why you can read forumers here saying they need to get police clearance in other country which they stayed/worked in recent time. Well, anyways this is my understanding. Cheers!