Excerpt from my modified file

Code:
   <hordes>
      <end_target_faction_bonus int="600"/>
      <start_target_faction_bonus int="300"/>
      <farming_level_bonus int="50"/>
      <shared_target_bonus int="10"/>
      <disbanding_horde_bonus int="-400"/>
      <starting_region_bonus int="950"/>
      <horde_target_resource_bonus int="1000"/>
   </hordes>
In the context of M2TW, where hordes do not disband upon taking a settlement, I was trying to get the mongols to act intelligibly and made several changes to this piece of code during a running campaign to observe behavior.

The maximum and minimum (working) bounds for all values seem to be -1000 to 1000. A value outside this (upper) range seems to make it do nothing (act similar to -1000)

horde_target_resource_bonus seems to refer to how prioritized the horde_targets are in descr_regions. Higher values mean that the horde will move towards and attack those targets with higher priority.

starting_region_bonus refers to how important it is to attack the region they spawn in and how much they want to attack it. Having this value higher than the horde_target_resource_bonus seems to make the horde inactive after taking the starting region and wander aimlessly around it (if they have raided rather than taken it)

shared_target_bonus refers to targets shared by other hordes. As there is only 1 horde_target variable which cannot be further specified to my knowledge, this means that all targets shared by mongols and timurids incur this modifier to their priority. Note that if you set it to a negative value (as per default), ensure that their starting region bonus will stay below the horde_target_resource_bonus after modification.

start_target_faction_bonus and end_target_faction_bonus seem to indicate the bonus added to continuing to attack the faction owning the starting region and the ones owning the other horde_targets.

While not completely clear on all the variables, from my testing the above posted ones should ensure general reliability of hordes.