Regular & Fill Servers

Sorry for a trivial question..
If I have 4 Servers, A & B which are Regular Servers, and C & D which are Fill Servers, the downloads are first tried on the regular servers and then on the fill servers. However, what about the priority between the regular servers and between the fill servers? Is that determined by the order in the Server list or random?
For Example, in the Server List, I have them listed in this order:
Server C (fill)
Server D (fill)
Server A (regular)
Server B (regular)
Would the priority order be A, B, C, D or would it be (A or B random), (C or D random) ?
If I have 4 Servers, A & B which are Regular Servers, and C & D which are Fill Servers, the downloads are first tried on the regular servers and then on the fill servers. However, what about the priority between the regular servers and between the fill servers? Is that determined by the order in the Server list or random?
For Example, in the Server List, I have them listed in this order:
Server C (fill)
Server D (fill)
Server A (regular)
Server B (regular)
Would the priority order be A, B, C, D or would it be (A or B random), (C or D random) ?