Lets be honest, polybutylene was a horrible choice of pipe to put in mobile homes. How it ever passed state codes, I don't know!
If you aren't in a position to replace all the polybutylene pipe and need to replace sections or fittings with Flair-It fittings, you will need this part on the 3/4" lines only.
This sleeve fits inside the polybutylene pipe, then you insert the Flair-It fitting nipple.