This description intended audience are LMS developers. :)
Originally access levels were supposed to be defined by various letters. It was assumption made at LMS-0.4, but it never actually took place. Due to it's presence in 1.0, I've racked my brain bout using 64-character string. So, in 64-character long field, there is just 256-bit hexadecimal number. Each character might describe maximum of 4 bits (4*64 = 256, which is a number of valid combinations). Turning some level on effects in turning some bit on. Then, if "full access" has index 0 in lib/accesstable.php, 0 bit will be set to 1, so the number will be 1. Levels may have numbers (indices) from 0 to 255. This is not final boundary, because using more letters and chars you can expand to 6 bytes per char easily, which gives you 384 combinations.