It doesn't not matter - as long as the fitting is the same diameter as the O.D of the pipe. For instance: With one 1/2" SharkBite fitting you can use 1/2" copper, 1/2" CPVC and 1/2" flex tubing at either end.

Edit: Sorry, misread the post. Thanks MGD77 and Tom for correct response..

The plastic insert is only needed on plastic tubing such as pex or quest. No need for the plastic insert on copper or cpvc. The insert is used to keep the soft plastic pipes round in the fitting. Hope this helps. Also, if you are asking which way the insert goes, the flanged end goes into the sharkbite fitting, and the other end will go into the pipe.

[wQUOTE=;][/QUOTE]Will this help? (see image)