Where and when do I use a gas valve

See Clause 5.2.7.1 A gas shut off valve (manual isolation valve) is required for every appliance. The valve must be certified and comply to AS4629 or (equivalent) must be quarter turn operation and open / closed position must be clearly marked.
The valve must be readily accessible and if the location is not VERY obvious it must be labelled (I would do this any way) The valve must be prior to the inlet of the appliance (see Clause 5.2.7.3).