Tees are described as, run x run x branch -

With runs being the parallel inserts and branch the perpendicular insert.  

Pex Insert Fittings are sized by the inside diameter (ID) of the tubing.  We have listed the Inside diameter (ID) and the outside diameter (OD) to help you select the right size fitting

Call Size  3/8" ID fits 1/2" OD

Call Size  1/2" ID fits  5/8" OD

Call Size  3/4" ID fits  7/8" OD

Call Size  1" ID fits 1 1/8" OD