They release a certain portion for each class for each ecr. Meaning that even if u are starting ecr at a later time, there a certain portion of slots for each class reserved for ur batch of ecr.
That's for 1st yr case as most of the units are common. When chiu come to yr2/3, graduating students will have a slight advantage when come to choosing unit. sae if u are AnF student, u needa do audit if not cannot grad, however all audit slots are full, the sch admin side will die die slot u into 1 of the audit class. but if u are y2 want to do audit but all audit class are full, sch admin will tell u do other units and leave audit to yr 3