Related Rule
Australia
Practice Relating to Rule 21. Target Selection
Australia’s Defence Force Manual (1994) states:
Objects and axes of attack should be chosen to minimise collateral damage wherever possible. Where a similar military advantage may be gained by attacking any one of several military objectives, the attack should be made against the objective which is likely to cause the least collateral damage. The same principle applies to choosing axes of advance or attack where more than one practicable and reasonable axis is available. 
Australia, Manual on Law of Armed Conflict, Australian Defence Force Publication, Operations Series, ADFP 37 – Interim Edition, 1994, § 552; see also Law of Armed Conflict, Commanders’ Guide, Australian Defence Force Publication, Operations Series, ADFP 37 Supplement 1 – Interim Edition, 7 March 1994, § 957(f).
Australia’s LOAC Manual (2006) states:
Objects and axes of attack should be chosen to minimise collateral damage wherever possible. Where a similar military advantage may be gained by attacking any one of several military objectives, the attack should be made against the objective that is likely to cause the least collateral damage. The same principle applies to choosing axes of advance or attack where more than one practicable and reasonable axis is available. 
Australia, The Manual of the Law of Armed Conflict, Australian Defence Doctrine Publication 06.4, Australian Defence Headquarters, 11 May 2006, § 5.57.
The LOAC Manual (2006) replaces both the Defence Force Manual (1994) and the Commanders’ Guide (1994).