I'm the father of two D-III guys, one having transferred from a D-II. At D-III's, you don't sign anything (a la NLI) as you do at D-I's and D-II's. Coach's guarantee of roster spot in our case was made orally, but prior players/parents told us that was as good as gold.
JT and others have described it pretty well.
Nowhere, at any level, is there much of a true guarantee. Even with scholly's, they're one year renewable, at the option of the school. Smart coaches honor their verbal commitments that the scholly will be there for four years (when my oldest red-shirted at his D-II, the coach said "of course we'll honor his scholarship for a 5th year"), but players tire of watching so a fair number transfer when it's obvious PT won't be happening much.
After day one, it's what you get done on the field, when you get your chance, while keeping the grades at a satisfactory level, at every level.