There are some additional guidelines that determine the layout, positioning and visibility of a pipe marker.
• The minimum letter height is 7mm and symbols must be at least 20mm high
• The minimum height of the flow direction indicator (arrow) is 10mm
• Pipes that are visible on two sides require (pipe) markings on both sides
• Pipe marking must be placed on imprtant places such as tap and branch points, valves, on either side of walls and other zone boundaries and at the beginning and end of pipe racks, etc.
More details can be found on the official ISO 20560-1 website: https://www.iso.org/standard/71570.html