Search Flows can be configured to auto-select one or more source. If this is used then the sources filter will be hidden from the filter panel in the main application.
These allow users to auto-populate metadata, entities and data values. They work as if a user clicked on one or more filters. These can be thought of as static filters. E.g. your search flow always applies 'classification = invoice'.
These allow you to analyse the query string and extract and select matching filters.
For example, you can map stw-brighton on the site code filter. With the use of smart synonyms, you can map 'working up a ladder' to the document type 'Working at Height'.