What is the drain line for?

[Lotatom]

Not familiar with softener with a drain line attached. This is how the builder left it for future install. What is the blue drain for?

 

[Bannerman]

Since the Blue Pex line is currently connected directly to the active supply piping, I anticipate it is not a drain line, but maybe a supply line to outside hose spigots or other faucets which you may not want to feed softened water.

A softener does utilize a drain line for elimination of hardness ions (mainly calcium and magnesium). As such, the softener will typically discharge to the home's drain system, which is normally routed to a municipal sewer or a private septic system.