The Filters button allows you to create, rename, delete and assign filters for the selected client.
Create - allows you to create a new filter, or sub-filter for the selected client. Without any filter selected the create button will create a new top level filter, to create a sub-filter, select the appropriate filter, then select create.
Assign - as the name suggests, this allows you to assign a filter, for a document that does not have a filter assigned, or to changed the assigned filter. Select the appropriate document, then select the assign option for filters and you will see this window to allow you to select the appropriate filter. By selecting one document and holding down the shift or control key you can select multiple documents to enable assigning filters in bulk.