When configuring a rollup field using the Count aggregation, the only option available is to count the records themselves.

Customers would like the ability to count unique values on the records.

For example, say I have Contacts, Projects, and Participations; Participations being an intersect between Projects and Contacts.

Currently, I cannot create a rollup on Contact which shows the number of Projects that the contact has participated in. I can create a rollup that counts the number of TIMES the contact has participated in any Project, but if the Contact participates in the same project twice, that will essentially double count it.