LE mode

There are a lot more than 2 modes. You can do 4 and 4, 5 and 3, some special 7LUT functions, etc. They're all dependent on your logic which is better. It's very difficult to control, and probably the last thing you want to be doing. (Stratix size devices would never get done if you had to worry about what size LUT all your logic was going into). There are occasional one-off reasons a user might need to force it, but in general let the tools take care of it for you. Instead worry about high-level settings like synthesis for Area, Speed, or Balanced, for example.