This limits the concurrency of extra searches to a percentage of the available cores. For example if the machine has 4 processors and this is set to '75' then a single search can execute at most 3 extra searches concurrently. The value is always rounded down if set to '99' on a 4 core machine we will use 3 processors.
A minimum of
1 processor will always be used.
50% of the available processors:
Use all but one of the processors (on a machine with 100 or less processors):
Uses 1 processors for extra searches no matter how many processors the machine has.