The colleges will have explicit directions on the steps to take and the deadline to submit the CSS application and possibly securely upload documents into IDOC.
CSS is far more detailed than FAFSA so set aside some time if required. If parents are divorced, the way things are filed they can not see each others information, but it is all tied to the student.
This is typically required with colleges providing significant financial aid or meeting full need. The college can use any formula they want to determine need and may not care about ability to pay. Some colleges will ask additional CSS questions - like UMiami wanted to know the cars we drive. Those aid packages using CSS data may not reflect what FAFSA comes up with. Also, even colleges that meet full need and say no loans, you may need a loan if they think you can afford $30,000 per year and you do not have a way to actually pay that much.
CSS is managed by College Board (SAT and AP Testing).
Here is their list of colleges for this year:
https://profile.collegeboard.org/profile/ppi/participatingInstitutions.aspx
IDOC info: