Our Terms and Conditions of Service require that accurate contact information is provided for all memberships; free speech is a responsibility as well as a right and part of that responsibility is that you may be held accountable for what you say. Supplemental verification is any step we take to verify your contact information above and beyond our usual policy of trusting what you enter when you sign up for and use our service.
There is a huge correlation between people who provide fake contact info and problems like payment fraud and illegal content. So if during the ordinary course of providing service we find a clear and specific reason to question whether accurate information has been provided, then we get into supplemental verification. To give an extreme example, if you tell us you're John Smith from Ontario, Canada, but pay with Bob Jones' credit card with a billing address in California, and you're logging in from Nigeria, we're probably going to give that a second look.
Depending on the situation, we may inquire about some or all observed discrepancies. That's usually enough to take care of it. In a handful of very rare cases -- about 0.01% of members -- we wind up asking for ID. In those extreme cases, generally any government-issued ID, like a driver's license or passport, is suitable for that purpose, but that comes up so rarely that we're very flexible in handling it on a case-by-case basis.
Typically any problem in this area is easily resolved. If not, it's for one of two reasons. Most of the time the membership was blatant fraud (stolen credit card, illegal content, etc). But sometimes the person involved says something to the effect of "I know your policy, but I don't agree with it, so I won't follow it." The nature of our Terms and Conditions of Service is that if someone refuses to abide by the Terms and Conditions, we will refuse to provide the Services, so that is not an ideological position that is compatible with use of our hosting. Importantly, it may also impair the ability to get a refund. (So if you're thinking about signing up with fake contact information, please don't. It'll eventually end in tears. Yours.)
Supplemental verification is also triggered anytime someone claims they are a member of our service but has neither the login information nor access to the contact email address. In those cases, we skip straight to asking for ID to establish the rightful member. (And naturally anyone who provided fake contact info and winds up in that situation is pretty much out of luck.)