There are three main components to petitions submitted to USCIS: filing fees, government forms, and Supporting Evidence. Supporting evidence is generally documentation to support your immigration case (e.g. birth certificates), eligibility evidence (e.g. permanent resident card or passport), relationship evidence (e.g. photos) and any documents corroborating information the applicant provided. Supporting documentation is required and is looked at very closely by USCIS.
Which supporting evidence you will need to include with your petition depends on what type of case you're filing as well as your unique circumstances (i.e. previous marriages, criminal history, etc.). RapidVisa supplies a customized list of supporting evidence to include with each customer's petition based on their answers given in the questionnaire.