Syntax: decimal number
This gives the minimum and maximum robots.txt
Crawl-delay values to allow; values found outside this range will
be changed to the appropriate minimum or maximum.  -1 means no limit.
The defaults are 0 and 10.  These values can be used to set reasonable
bounds to sites' Crawl-delay values.
Note that a Crawl-delay seen (modified to these limits) is only
used if Robots robots.txt is Y, and overrides Walk
  Delay (here).  Thus, to use the greater of
robots.txt Crawl-delay or Walk Delay (e.g. if the
walker is bandwidth-limited, and sites' robots.txt delays are
to be followed), keep Min equal to Walk Delay.  To always
use Walk Delay but still respect other robots.txt
directives (i.e. just ignore Crawl-delay), keep Min and
Max equal to Walk Delay.