How to get numbers between 2 numbers?

If by "between" you mean "including the lower and upper values" then this would work for you:

if(
  ri!lower > ri!upper,
  fn!tointeger({}),
  fn!enumerate(ri!upper - ri!lower + 1) + ri!lower
)

...where ri!upper and ri!lower are integer values.

If you want to exclude the lower/upper values then that would be a small tweak to the above code.

If by "between" you mean "including the lower and upper values" then this would work for you:

if(
  ri!lower > ri!upper,
  fn!tointeger({}),
  fn!enumerate(ri!upper - ri!lower + 1) + ri!lower
)

...where ri!upper and ri!lower are integer values.

If you want to exclude the lower/upper values then that would be a small tweak to the above code.