The numbers are not really an industry standard, but you generally find one of these three:

1. The number is directly derived from the spine deflection rating for that shaft, such as on Carbon Express, Beman or Easton carbon shafts:

400 = ~ .400" deflection (less stiff)
350 = ~ .351" deflection
340 = ~ .339" deflection (more stiff)

In this case, a higher number means a "weaker" or "less stiff shaft"

2. The number is derived from the stiffness of the arrow such as PSE Carbon Force arrows:

100 = weakest / lightest shaft
400 = stiffest /heaviest shaft

3. Some mfrs simply give you a number that is a relative guide for what poundage bows the shafts are appropriate for. Such as Gold Tip shafts:

3555 or 35/55 = lighter/weaker arrow for bows between 35 and 55 pounds
5575 or 55/75 = heavier/stiffer arrow for "" "" "" 55 and 75 pounds
6595 or 65/95 = heaviest/stiffest... etc..