The rule your looking for is 1.12.8, highlighted relevant parts.
Quote:
1.12.8 Loser Withdraws After the result of the combat has been worked out (either because all defending units were destroyed, or the attack stalled, or through a result roll), then the loser is broken and must withdraw, and formations on the winning side receive Blast markers for the casualties they suffered.
If the loser was already broken when it was assaulted then the whole formation is destroyed, and ALL units in the formation are removed from play as casualties. If the losing formation is not broken then it becomes broken and may withdraw (see 1.13.3).
Once any additional casualties have been removed, any surviving units on the losing side must make an immediate withdrawal as explained in the rules for broken formations later on (see 1.13.3).
Finally, any formations belonging to the losing side that were in a position to have lent support (ie, they were within 15cms of an enemy unit in the assault) receive one Blast marker each, even if they did not actually lend support. These Blast markers represent the detrimental effect on morale of seeing friends defeated in an assault.
Also remember that supporting formations (on the losing side ) only get a BM if they were within 15m of a unit that was "directly involved" in the engagement, not from being within 15cm of a unit that was not "directly involved"