Use Mask

Key Use Mask in Number Is Inside Fn refers to an optional boolean that can be used to instruct the function to refrain from checking whether certain preselected variables out of the input variables x₁ , x₂ , ... , xᵤ are within their corresponding intervals and instead assume they are.
If missing, the default is FALSE, which means all input variables x₁ , x₂ , ... , xᵤ are checked normally.
If set to TRUE, the first input variable x₁ acts as the mask that sets which exactly of the remaining variables x₂ , ... , xᵤ have their checking bypassed.
Specifically, the mask is implemented through the binary representation of x₁

For example, assume there exist 4 input variables x₁ , x₂ , x₃, x₄ and x₁ = 5
The binary representation of the number 5 is 101
The digits 1 , 0 , 1 of the binary number 101 are assumed to correspond to the variables x₂ , x₃, x₄ in that sequence.
In detail:

1 → x₂
0 → x₃
1 → x₄

Those variables that are assigned the 1 are assumed to lie within their corresponding intervals and therefore that condition will not be checked at run time.
So, the variables x₂ , x₄ are assumed to lie within their corresponding intervals.
At run time, only the variable x₃ will be checked.