Field Validation on data entry

Hi all
Talking about forms:
Maybe I misunderstood, but what does the flag “To use in the validation” in the lookup of each field?
I always understood that it meant “do not accept the field value if it is not satisfied by the lookup statement” but now I realize that I can enter any value in the field.
How can I influence the validation of the field according to the lookup statement?
I must repeat the checks for each field in the form?!?!?!
Any suggestion is precious. Thanks.