Quick answer
Choose armoured wherever the route offers no mechanical protection: direct burial in the ground, outdoor runs at risk of impact, and industrial areas with traffic or rodents. Unarmoured is appropriate indoors, in conduit or duct, and on protected trays. One detail worth knowing: single-core cables use non-magnetic aluminium armour (steel around a single core would heat up from induced currents), while multicore cables use galvanised steel. Our cable armouring guide covers the designations.