The NLI is a documentary agreement signed by the athlete and the school. Signing Day (or Period) is the time during which the NLI may be signed.
NLI schools do not have slots. They have athletic scholarships. Coaches at NLI schools should be advising recruits whether they are being recruited for athletic scholarships or are being invited as walk-ons .
Ivies have slots. Coach should tell you after Admissions has confirmed a positive academic pre-read, or you can ask, whether the coach is allocating a slot to you.
There is a divergence of opinion on the forum about the level of support attached to an Ivy slot.
My opinion is that there is either support or no support. Others say or suggest there are slots with support for a likely letter, which are more solid than slots without support for a likely letter, or no support.
In this regard, see this comment: “A second tier of “soft-support”, not included as part of the formal number of supported athletes would really undermine the whole process.”
http://talk.collegeconfidential.com/athletic-recruits/1786857-ivy-league-school-or-mit-p4.html
And this: “My opinion is that a recruited athlete on the list is just as solid whether or not they get the letter in the mail.”
http://www.tier1athletics.org/category/ivy-league-academics/academic-index-ivy-league-academics/