Documentation for Crowd 1.0. Documentation for other versions of Crowd is available too.
When you map multiple directories to an application, you also need to define the directory order. This is important in case the same user exists in multiple directories. When a user attempts to access an application, Crowd will search the directories in the order you specified, and will use the credentials (password, etc) of the first occurrence of the user to validate the login attempt (see diagram below).
To specify the directory order,
(Note: in Crowd, users are known as principals.)
Screenshot: 'Application---Mapped Directories'
How it works
Let's assume that JIRA has been set up as a Crowd application, and has been mapped to two directories, 'Partners' and 'Customers', in that order (as shown in the above screenshot).
Here is what happens when a user attempts to login to JIRA: