Everything that decides the case. Vetting must be completed, the gift made when instructed, an EB-1 or EB-2 determination reached subject to visa availability, and any interview or further document request answered. The applicant must be eligible for lawful permanent residence and admissible. The fee buys processing of an application, not an outcome.
List the open conditions on one page
Write each remaining condition as a line with a status: complete, in progress, or unknown. Include admissibility questions such as past refusals, criminal history, or prior overstays, since these are exactly the issues an interview may explore and none of them are waived by participation. Reviewing that page with counsel before booking anything converts a vague sense of progress into a specific list of what would have to happen first.
Give the admissibility questions their own line rather than folding them into a general sense that everything is in order, since they are decided on their own facts and are not affected by any payment. A prior refusal, a period of unlawful presence, a criminal matter however old, or a misrepresentation in an earlier application each deserve to be raised with counsel at the outset, when options exist, rather than at an interview. Applicants often omit these because they were resolved long ago or because a previous adviser said they were unimportant; the safer approach is to disclose them to counsel and let counsel decide what matters.