Here is the quote from the rules
Quote:
1.12.6 Supporting Fire
Both sides may call upon support unless the defender has been wiped out or the attack stalled as described above. Calling on support allows units from other formations to attack with their firefight value if they are within 15cm and have a line of fire to an enemy unit directly involved in the assault. In this case ‘directly involved’ means belonging to the attacking or defending formation(s) and in a position to attack. This rule represents units from both sides that are not directly involved in the assault lending supporting fire when they see their friends coming under attack. Units from formations that are either Broken or Marched this turn cannot lend support.