Recommended Practice: A document that is similar in content and structure to a code or standard but that contains only nonmandatory provisions using the word should to indicate recommendations in the body of the text
NFPA 53 Recommended Practice on Materials, Equipment, and Systems Used in Oxygen-Enriched Atmospheres, 2004 ed. https://www.nfpa.org/assets/files/AboutTheCodes/53 /53-F2009-ROP.pdf, (Recmm Prac: on Mtrl, Equip and Sys Used in Oxy-Enriched Atm - Amer) {209 NFC}