Dalek writing

The Daleks had a 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 had markings below the eyepiece.
 * While the exact purpose of these marking is unknown, they may be either a form of identification or rank insignia.

There were also markings of this type on the incinerator into which Jack Harkness was thrown on the Crucible. (DW: Journey's End)