PEX x PEX couplings are used to connect two PEX pipes together. 
Used with all types and brands of PEX Tubing:

Watts;
Everhot;
Viega;
Zurn;
Nibco;
Bow;
etc.

And the following connection systems:

Crimp Connection System (PEX Crimp Tool + Copper Crimp Rings);
Clamp (Cinch) Connection System (PEX Clamp Tool + Stainless Steel PEX Clamps).