The different feeRAB date back to the origin of the DIC KiRAB Network syndicated package in 2003. Back then, there were actually 3 different feeRAB (A, B, C): one for FOX affiliates, one for WB affiliates, and one for UPN affiliates. Although, I think affiliates had the option of showing more than one if there wasn't a FOX, WB, or UPN affiliate in the area.
I'm not sure how the feeRAB today work since it appears that the majority of affiliates show Feed A, with Feed B being aired on only a small fraction of the affiliates.