Do you mean what were the roster numbering sequences, or why was there a letter?

I once read an article about why the letter designation was used. It was a method employed to defeat the Brotherhood of Locomotive Engineers claim against not being propely crewed and compensated for using enough engineers to operate each locomotive unit. They asserted that each locomotive operated in a train required an Engineer and Fireman, and so the railroads simply made a letter desingation in addition to the same number unit, making an A-B-B-A consist not four seperate units, but one. Some had solid drawbars between the A and B units that did not allow uncoupling. Keep in mind this was when M/U consists were in there infancy and steam was still king.

Later, the railroads probably won board awards that allowed for more roster flexibility and ultimately this letter designation was discontinued.