Yellow teflon tape is the gas-rated grade of PTFE thread seal tape, denser and usually thicker than white at 0.55–0.75 g/cm³. Gas seeps straight through a low-density white water tape, which is why most gas codes call for the yellow one on natural gas, LPG, propane and butane.
High density PTFE TAPE for gas pipe are used for high pressure systems,for critic and special purpose. According to MIL-T27730A, UL.

High density PTFE TAPE is thicker and heavier tape. it’s easy to apply and creates a more effective seal with less tape,and no need for pipe dope. It’s used on all threads made by galvanized steel,iron,brass copper,aluminum,stainless steel,polyethylene,polypropylene,pvc,cpvc,abs,fiberglass and more.