IDOC doesn't verify anything. It is simply a service that submits copies of your financial documents electronically to the schools that subscribe to the service. In a way, though, I guess you could consider sending all those documents as "verifying" the information on your CSS Profile form. But people aren't "flagged" for that. All schools that use the CSS Profile and IDOC require all applicants to send those documents.
FAFSA flags about 30% of applicants. Sometimes it can be at random, sometimes because of certain information you provided on your FAFSA. It's hard to tell why. For FAFSA verification you don't need to do anything at all until you are contacted by your college. They will tell you what documents they want to see, and you will send copies of those directly to the school that asks. Some schools will ask for them even if you're not flagged for verification by FAFSA, some schools won't ask even if you are flagged. But wait until they contact you and tell you what they want from you.
Schools are required to verify something like 30% of their FAFSA applicants by the government. But once they've met the required number of verifications, they can choose to not request anything further from students who were flagged by FAFSA when they submitted their app. For example, my daughter's FAFSA was flagged last year, but the college never requested any documentation from us so we didn't have to do anything further.