NOT exactly an expert here but i believe you can sign at any / all levels, D1, D2, D3, NAIA & JC. This is not a solid deal by the way - either party the school or the student athlete can back out because intent is not a signing - it is only a promise to sign - like an engagement ring - until you walk down the aisle and say I do it is an "I maybe".
see definition i found: "A letter of intent is typically written by a student who is looking to sell himself to an undergraduate college/university, a graduate school, an athletic department at a college/university, or a business that provides an internship. The purpose of the letter of intent is a sales pitch. You are selling yourself and your abilities. "
one bad thing about this can be that a coach in a college will then stop looking for that position, but what happens if that student athlete back out - they have to now start all over.