Your full immigration and travel record: prior visa refusals, removals, overstays, arrests or convictions anywhere, and any earlier applications containing information you would now correct. Admissibility is a genuine condition of the case and is not waived by the program. Raise these with counsel before payment, since the processing fee is not refunded if a problem emerges later.
Assemble the record before the portal stage
Collect passports covering the last several years, entry and exit stamps, copies of previous applications and decisions, and court or police records where they exist. Supporting documents are later filed with Form I-140G through the official portal under the instructions USCIS issues, but the honest review happens earlier, with counsel. A problem identified now can sometimes be addressed; the same problem surfacing at an interview is far harder to manage.
Assemble the record before anyone forms a view, since an assessment given without it is comfortable and unreliable. Passports covering the last several years, entry and exit stamps, copies of previous applications and their decisions, and any court or police records are the material that determines admissibility, and admissibility is a condition of the case rather than a formality attached to it. Where a matter was resolved long ago or a previous adviser said it was unimportant, disclose it anyway and let counsel decide, because the cost of raising it now is an hour and the cost of it surfacing later can be the whole case.