An option for the support attachment or rules attachment.

<option name="players" value="Russians"/>

Values are valid player names, restricts bonus to this player if left blank, it applies to no one, so be sure to fill it unless you want it not to activate at the beginning of the game

<attatchment name="supportAttachment1" attatchTo="artillery" javaClass="games.strategy.triplea.attatchments.UnitSupportAttachment" type="unitType">

  <option name="unitType" value="infantry"/>
  <option name="faction" value="allied"/>
  <option name="side" value="offence"/>
  <option name="dice" value="strength"/>
  <option name="bonus" value="1"/>
  <option name="number" value="1"/>
  <option name="bonusType" value="arty"/>
  <option name="players" value="Russians"/>
  <option name="impArtTech" value="false"/>


Rules attachment which can be used with directOwnershipTerritories, directExclusionTerritories, directPresenceTerritories, or any of the other territory lists, to customize which players are required to own/occupy the territories. (veqryn)