In June 1910, the ARA accepted the proposed classification of cars that had been drawn up at its request by the Master Car Builders Association. The complete list was published at the time in various trade journals including Railway Age Gazette 1910 vol. 48, beginning on page 1620. This can be viewed or downloaded at https://babel.hathitrust.org/cgi/pt?id=njp.32101049000712;view=1up;seq=1662

There were four classes for “General Service Freight Equipment Cars” –

XM Box Car
XA Automobile Car
XF Furniture Car

XV Box Car Ventilated

It defines XA as “Box car of similar design to general service car, having exceptionally large side doors or end doors”, while XF was as per XA “except unusually greater capacity in cubic feet”.

Rupert Gamlen
Auckland NZ

One document I saw (http://www.nakina.net/other/aartype.html) derives its list of AAR Mechanical Designations from Official Railway Equipment Registers starting in 1917. The range of years to which the designations apply also are listed. 

Unfortunately, the list does not go back before 1917 so the word "carriage" does not appear in the descriptions. Note that "XA" evolved and overlapped ""XAR" and "XF" appears to have lasted into the mid-1960s.

Bob Chaparro

Hemet, CA