For stuff that goes beyond just being a US Citizen or Permanent Resident with no criminal record:
If they want you to do an SF-86 with the US OPM for a Secret or better clearance, it really means "if you even THINK you have something negative in your personal background, don't waste our time or yours in applying"
(reference: I'm one of the peoples whose full set of personal data was stolen in the OPM data breach)
https://en.wikipedia.org/wiki/Office_of_Personnel_Management...
Not really in any way SpaceX specific, but this is something you will commonly see if applying with any large defense contractor/US-based aerospace industry firm.