There are a few 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 indicator (arrow) is 10mm
• Pipes that are visible on two sides require (pipe) markings on both sides
• Pipe markings should be placed at key locations such as junctions, valve breaks and branch points, on both sides 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