My understanding is that NASIS is a necessary step during the Officer recruitment process as Officers require a security clearance as a requirement for holding their commission.
When I went through the process I filled out the NASIS and they were able to pull my existing clearance as I already had one from my last job as a civilian contractor for the Navy, which I left for my current job < 2 years ago, so they were able to pull it successfully.
Some communities such as IW (Intel, Crypto, IP, etc) do require additional security clearance prescreener interviews and paperwork as they require TS/SCI.
The NASIS is the Navy’s way of kickstarting your SF-86 for pulling your existing clearance or triggering a new investigation… I would check with your recruiter if you have more specific questions. Is there a specific community you are applying to?