The Totality Concept was a widespread and highly secretive series of experiments and projects run by the US government and military. There were many, many subprojects under the umbrella of the Totality Concept, which are roughly organized as noted below:
Most Totality Concept installations (and all of those with mat-trans units) have areas that are accessible only by people with a specific security clearance. Clearances were a letter/number combination - A, B, or C, and a number from 1-19*: A-4, B-12, C-5, etc. Someone with a "B" rating is higher than any "A" rating, and a "C" rating is higher than either "A" or "B" - for example, a B-2 rating is higher than A-10, and C-5 is higher than B-12.
While numbers higher than 19 appear in later books, B-12 is the standard clearance required for the mat-trans in the early books, and B-19 is the highest number that appears at all (Doc Tanner once said that this level is what the "nabobs from the Pentagon... privy to Whisper" would have had (Dectra Chain). Thus, it is reasonable to assume that this is the highest clearance rating for a given letter, and that the later numbers were simply made up.