Secondary marking is defined as the marking used within a major system or location to identify components such as valves, hydraulically operated components, local tapping points used for sensing equipment, probes, etc. A character height of 50 mm (1,969 in) to 150 mm (5,906 in) should be used. Smaller sizes may be used when the specified size is impractical.
Source: API RP 17A, Design and Operation of Subsea Production Systems—General Requirements and Recommendations, Fourth Edition, Reaffirmed 2011. Global Standards