Folder Filter Expressions are a dynamic form of security that uses an advanced syntax string to determine what users should be able to access which documents in a folder.
A user with the Manage Filter Expressions privilege for the repository and the Write Entry Security entry access right for the folder can apply a filter expression specifying what criteria should be used to determine access to the entries; when a user opens the folder, the Server will use this criteria to determine what documents they should be able to see.
Folder Filter Expressions work with other security settings, and will not permit access that has been denied by another security setting. Because they are dynamic, Folder Filter Expressions allow you to vary access to documents within the same folder without needing to maintain separate entry access rights for each document.