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NBC 4.1.8.11.(12), calls for a 20% increase in base shear for 5- and 6- storey structures when periods are determined using 4.1.8.11.(3)(d), i.e. through "other established methods" rather than the formula in 4.1.8.11.(3)(c).
It a T you have you have entered yourself is greater than greater than the Ta from 4.1.8.11.(3)(c) we 4.1.8.11.(3)(d) is being used and the 20% surcharge is applied. A decrease in the period is assumed to mean that you are just adjusting the height used in 4.1.8.11.(3)(c), as no advantage is accrued.
The increase is applied to each orthogonal direction independently. When applied, the equation for V shown in the Equations section of the Load Generation Details Output includes the 1.2 factor. In the unusual case that it is applied in only one orthogonal direction, two equations are shown for V, one for each direction.