Until Errata'd, Cluster Artillery, and a few others, would use the "Shot from Above" Hit Location Table (which was very similar to the Punch Table). It was deleted with the 5th Printing of Advanced Units and Equipment.
So, it's only used when an Infantry unit in a Building is shooting at a Mech in the same Hex as they are, while they are higher than the 'Mech's base (i.e Level 1+). (Total Warfare pg 175).