Are you asking about PEX fares on airlines? If so they are discounted return fares available on many (not all) flight routes. Once the ticket has been purchased, changes can not usually be made. The lack of flexibility about dates, with other restrictions on length of stay often means that Pex fares are not suitable for business travellers.
If you are referring to PEX tubing for compressed air, I don't believe it has been recommended, maybe someone else here knows. Another thing is PEX is not UV stabilized, so it is not supposed used outside for prolonged exposure.