In which Situation instead of three phase 4 wires LT Cable (Full Neutral), a three phase 3-1/2 Wires LT Cable (Reduced Neutral) is used?

The size of the neutral conductor is calculated based on the network balance and how much current will pass through neutral cnductor . base on my previous works I can say for power circuits where most of the loads are three phase load I use50% reduced neutral but if you have in additional to the three phase load some dimmers , variable speed drives or industrial electronic devices it is required / or recommended to use100 % neutral size and for some minor case we can use200% neutral sizw 

In case where half of your connected load is three phase induction motor or three phase primary delta winding transformer. In general, where majority of your load does not require a neutral.