A spec sheet for every guitar in your case. Save the setup once, and the numbers are right there the next time you need them.
They're saved privately in your own browser using local storage. Nothing is uploaded to a server, so your profiles stay on the device you created them on. The trade-off: clearing your browser data removes them, so open a guitar and use Print / Save as PDF now and then to keep a backup copy.
No. There's no sign-up and no login. Add a guitar and it's saved instantly on your device.
Anything that helps you recall a setup: bridge type, scale length, string gauge and tuning, action at the 12th fret, neck relief, nut and pickup heights, and the dates of your last setup and string change. Every field is optional except the name.