Syncthing has settings that are available but not exposed in the usual folder and device configuration screens. The reason for not exposing these settings more prominently varies from them simply being rarely useful, to them being actively dangerous when used improperly.
Use care when changing advanced configuration settings and make sure that you understand the resulting effects and consequences.
The advanced settings dialog is accessed from the main menu:
Per Folder Settings
.. toctree:: :maxdepth: 1 :glob: ../advanced/folder-*
Per Device Settings
.. toctree:: :maxdepth: 1 :glob: ../advanced/device-*