Players use dice are used for three primary purposes: to determine damage, to determine success at a task, or for combat resolution.
d4 - Used to determine damage for smaller weapons like daggers, caltrops, brass knuckles, etc.
d6 - Used to determine damage for simple weapons, arrows, bolts and other hazards (like falling)
d8 - Used to determine damage for crafted weapons like swords. It is also used to determine success for tasks not covered in the rules.
d10 - Used to determine damage for large crafted weapons like halberds and two-handed swords.
d% - This refers to using two 10-sided die to determine success with certain tasks, usually special abilities such as Pick Locks or Catch Object. One die is treated as a tens-digit, the other as a ones digit. Results range from 01 - 99.
d12 - Used to determine damage for unusual monster attacks and for psionic combat.
dB% - This refers to using two 12-sided die to determine success with certain tasks for higher-level players for advanced abilities. The result of one die is multiplied by 10 and added to the other die. Results range from 11 - 132.
d20 - This die is used specifically for melee, missile, projectile and magic attacks.
The GM uses these die for mostly the same uses. At his or her discretion, other die may also be used. These other die are predominantly used for random generation purposes, for example, generating a random monster, etc. These include, but are not limited to:
d66 - Using two six-sided dice, multiply the result of one die by 10 and add the result of the other die.
Others include d3, d14, d16, d24, d30 and d100.
dB% is something I used to do for Thieves with high ability scores or for thieves at high levels. I added a bunch of abilities available only at level 12 and above like ventriloquism, throw voice, climb sheer surfaces, etc. I think Mr. Gygax himself suggested an arrangement like this at some point, but I don't recall seeing anything more concrete.
Since special abilities are open to any class generated, I plan on using dB% to represent characters that have high ability to perform one of these tasks - a 16 or above. Since the core mechanic of the system will consistently be a roll-over mechanism, some of the scores will be listed as >100.
Next post, I should include the Skill Tables.