Dalek writing

The Daleks have a (seldom-revealed) form of writing.

Overview
During the 22nd century invasion of Earth, the Daleks made notices around London locations with uncertain meaning. (DW: The Dalek Invasion of Earth)

Post-Last Great Time War Dalek Markings
All Daleks from a post-Last Great Time War era have markings below the eyepiece.
 * ''While the exact purpose of these marking is unknown, they may be either a form of identification or rank insignia.