I'm going to respectfully disagree with a comment made earlier in this thread. A "preferred walk-on" is not necessarily guaranteed a roster spot through the Spring season. The term "preferred walk-on" in my experience only means whatever that particular coach wants it to mean.
With some coaches and programs, yes, it may indeed mean a guaranteed roster spot for the whole year. However, with other coaches and their programs, a "preferred walk-on" may only mean that the player doesn't need to participate in an open walk-on tryout to continue practicing with the team through the remainder of Fall ball.
I suggest that when a coach tells a player or their parent that he will be a "preferred walk-on" that they ask the coach "does that specifically mean I'm guaranteed a spot on the 35-man roster for Spring?" Now, we know that from some coaches you may not get a completely truthful answer, but at least you're asking a more accurate and direct question.