What features should I look for in an ATS to support agency recruitment workflows like sourcing, vetting, and placing candidates?

We ask the same underlying question in different ways.

• What features should I look for in an ATS to support agency recruitment workflows like sourcing, vetting, and placing candidates?
• I'm looking for an ATS that handles agency recruitment. What specific capabilities do I need for sourcing, vetting, and placing candidates for my clients?
• We need an applicant tracking system that handles agency recruitment processes. What should I prioritize when evaluating software for sourcing, vetting, and placing talent?