The short answer to the OP: RCI was running out of 4-digit resort IDs to give to new member resorts so they started using letters.
Background:
RCI resort ID's used to all be numbers. As resorts joined RCI, they were assigned a member number, starting with resort # 1, which happens to be Fairway-Shawnee (yup, the Poconos had some of the resorts first traded through RCI). In the early years, RCI only put out a magazine-style Resort Directory and exchanges were obtained by phoning in. Adding digits or zeroes to the resort ID numbers was not complicated. It was just typed into the resort descriptions with each new year's Resort Directory.
When RCI went computerized and they offered online exchanging, all the resorts had 4-digit numerical resort ID's (IIRC). IOW, Fairway-Shawnee's resort ID# online was not # 1 - it was, as it is now, 0001 (again, IIRC). At some point, when the international resort ID #'s were reaching the 8000's and growing fast, RCI started using resort ID's that started with a letter in the first digit place.
(Note that, by this time, many resorts had multiple resort ID #'s. Older resorts had sometimes split into several resort regimes with separate management. Some resorts added new sections to older resorts. Some, especially those still in development or active sales, wanted to control where incoming exchangers could be placed within their resorts so they wanted a new ID # for each section. Ultimately, the number of new resort IDs needed was actually larger than the number of member resorts in the RCI system.)
Adding another variable digit to a computer field is not a simple matter. Remember when most computer systems only looked at a year as a 2-digit field and during the Millennium change '99 to '00, there were concerns? Changing all the member resorts' ID #'s to a 5-digit ID# (ex., making Fairway-Shawnee into ID # 00001), just to increase the total possible number of resort ID's available to fit in the system's Resort ID field was not a good option. However, potentially adding 26 letters (alpha digits) to the 10 numbers (numerical digits) to the acceptable form of data in RCI's computer systems' Resort ID fields would dramatically increase the possible number of discreet resort IDs available. And this was probably a much easier IT fix than making all the 4-digit fields into 5-digit fields.
If you do a quick search of all U.S. resorts on the RCI Resort Directory online, display the results, ordered by RCI's Resort IDs, and click through, you'll see that the numbers go up, one at a time, to #8897 (Holiday Inn Club Orange Lake North Village), then switch to A000 (WorldMark Granby Rocky Mtn Preserve). However, the earliest alpha RCI Resort IDs did not all begin with the letter A and they have not been added to the system in alphanumerical order either.
At this time, I see
U.S. resorts with Resort ID #'s: xxxx, Axxx, Cxxx, Dxxx and Exxx, but no Bxxx at this time, primarily for exchanging. Some make sense with the resort's name, such as Disney Vacation Club's, whose week-long exchanges are listed with Resort ID #'s DVxx. I don't know if other starting letters are used for other specific companies or for specific countries either.
RCI week-long rentals may show under any resort ID but some resort ID's appear to be strictly rental inventory:
Rxxx.
Short stays (3-nights, 4-nights, or 5-weekday-nights), either exchange or rental, appear with another distinct ID:
Sxxx.
Hope that makes sense.
Probably MUCH more info than you expected or wanted! LOL!