Evaluation of a list of numbers

If your function ILt( ) is written in way to work OK with single, scalar arguments (of dimension 1/time) and you want to call that function with a vector of values as its argument and expect it to feed those values one by one into the function, evaluate it and collect the results in a vector, then you have to use vectorization because that's exactly what vectorization is made for!
Depending on how your function ILt( ) is defined it may not be necessary to manually apply vectorization as Prime might automatically implicitly applies it, but I always suggest to manually apply vectorization in such cases.
After all, if the function was written to work with a single scalar argument, there is no reason whatsoever that it should also work with a vector as its argument instead! So its perfectly OK and natural that you have to tell Prime what you expect it to do with the vector argument.

Use the appropriate keyboard shortcut or the menu to apply vectorization:

BTW, the keyboard short you see in the tooltip help might be wrong, depending on the keyboard you are using. On my German keyboard its Ctrl-Shift-6 (or Ctrl-Shift-&) and not Ctrl-Shift-^ as the help claims it to be.