A binder is a folder that groups objects of different types (documents, messages, events,..) and different locations on the project.
The objects that are shown in the binder are links to the actual objects that are stored somewhere else in the project.
Different ways to use a binder:
1. Group different types of objects
A binder can be used to group different kinds of objects that are spread all over the project, for example, to prepare a meeting.
2. Give a group of users access rights to a few objects
A binder can be used to give a group of users access to a few documents. If you give these users (via a roll) access to the binder, they do not need to have access to the original object in the containing folder itself.
3. Access to documents with a certain status
The administrator can edit the binder settings and enable that only certain document statuses are shown in the binder.
This way you can give users access to a binder which contains multiple documents but only when the status of the document matches the filter on the binder, the document will appear.