Your customer list against a partner's. Two companies each load their customers and prospects, a clean room compares them, and both sides see shared accounts, overlapping prospects and white space. This is what partner and ecosystem teams mean, and it is the biggest of the three by spend. The products are Crossbeam, PartnerTap, Kiflo and WorkSpan. Nothing happens until a second company opts in.
The people inside one account. Who signs, who blocks, who reports to whom, drawn on the CRM record for an account you're already working. Strategic account managers mean this one, and the products are DemandFarm, Prolifiq and Altify, all Salesforce-first.
Who on your side already knows someone there. Not companies, people: which of your colleagues has emailed, met or connected with a named person at the account, and how well. This is what a rep or a founder means when they ask before the first touch, and it's the one meaning the partner tools can't answer at all, because a partner overlap is a lead rather than a path.
Nearly every bad purchase here is a buyer with meaning three shortlisting products built for meaning one.