How to get numbers between 2 numbers?

a!localVariables(
  local!numberOne: 17,
  local!numberTwo: 5,
  if(
    local!numberOne = local!numberTwo,
    {},
    a!localVariables(
      local!lowerNumber: min(
        local!numberOne,
        local!numberTwo
      ),
      local!upperNumber: max(
        local!numberOne,
        local!numberTwo
      ),
      local!difference: local!upperNumber - local!lowerNumber - 1,
      1 + local!lowerNumber + enumerate(local!difference)
    )
  )
)

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.