Subject to apportionment, residence, and other requirements of law and this chapter, OPM shall enter the names of eligibles on the appropriate register in accordance with their numerical ratings, except that the names of:

(a) Preference eligibles shall be entered in accordance with their augmented ratings and ahead of others having the same rating; and

(b) Preference eligibles who have a compensable service-connected disability of 10 percent or more shall be entered at the top of the register in the order of their ratings unless the register is for professional or scientific positions in GS-9 and above and in comparable pay levels under other pay-fixing authorities.