These are sensors used by the printer to identify the correct placement of the label, for accurate printing.
In order for labels to properly feed through the printer in the correct alignment, the printer has to be able to identify where each label starts and ends. To do so, the printer uses sensors to detect black marks or notches between labels. This allows the printer to position labels properly and to produce a well-aligned printout. In order for the sensors to detect the black marks or notches, please make sure that you select the correct Media Type as described below.
If you use BarTender, please press Print and click “Document Properties” underneath your printer’s name. Select the “Stock” tab and make sure that “Non-Continuous (Mark Sensing)” is set as Media Type.
You can adjust the settings on your printer directly if you do not use BarTender. For your Citizen CL-S631, please refer to pages 45 to 47 of your manual. Your sensor will have to be set to “Reflective” in order to recognize the labels correctly. For your Zebra GX430t printer, please refer to page 102 and/or page 107 of your manual.