Numbering defaults sets the way CallSwitch MT will assign network numbers to Extensions, Conferences etc...
Fetch least unallocated number (default):
This option takes the lowest number on the system that is not used by the system and assigns it to a new Extension you're trying to create for example.
Fetch next unallocated number:
If the last allocated number was assigned to extension 2010, the next IVR that you're trying to add for example will be given number 2011. The system will not try to give you an unallocated number 1022 for example
Use this option in case that you want to assign numbers in non-sequential order. If you create a new Queue for example, CallSwitch MT will assign it a number 2350, for example. And if you try to create a new Extension right after that, CallSwitch MT might assign it a number 9838, for example.