If you're building a requirements set based on discussions with a client they need to be captured in some form that allows the PM, the team and the customer themselves to see them clearly articulated. Once that's done, whatever document/tool is used must be baselined. By that I mean that the customer and the PM must formally agree on, and publish them. That's the only way the PM is going to prevent scope creep and resultant cost and schedule variances. The type of tool needed for baselining depends on the complexity of the project. It can be as simple as a spreadsheet or, if necessary one of the many more sophisticated requirements tracking tools available commercially. Whatever method is used, each requirement needs at a minimum to: 1) be clearly stated. 2) have it's source identified. 3) have it's linkage to other requirements identified.