Measurement scale markers display the measurement scale used in your document. You should set the measurement scale for a document before creating a scale marker. You can set the marker length in logical units, include a text caption indicating the length, and set the marker and caption color to black or white.
The scale marker is placed in the lower left corner of the image. The marker adds a layer group to the document, containing a text layer (if the Display Text option is selected) and a graphic layer. You can use the Move tool to move the scale marker, or the Text tool to edit the caption or change text size, font, or color.
For a video on understanding and using the measurement features, see www.adobe.com/go/vid0029.
You can place multiple scale markers in a document, or replace existing markers.