USB-C is the port, like the form-factor, pins all that stuff.
If there is USB 3.1 gen 2 interface behind this port, you get up to 10 Gbit/s performance and can only connect USB and DP devices, not TB devices.
If there is TB3 interface behind this port, you get up to 40 Gbit/s performance and can connect both USB and DP and TB devices.
TB3 interface is much more expensive to implement obviously.
P. S. Retina MacBook has USB 3.1 gen 1 (previously known as USB 3.0) interface behind its USB-C port if I'm not mistaken.
[doublepost=1454204510][/doublepost]AFAIK neither CPU nor logic board provides TB3 in Skylake generation of chipsets and CPUs. You have to install dedicated Alpine Ridge controller. And there are only controllers for 1 or 2 ports.
If Apple decides to give us 4 TB3 interfaces they will have to install two 2-port controllers (which is quite pricey) and dedicate 8 PCIe lanes to them (Skylake CPU has 16 lanes total, SSD will get 4, dGPU will take 4 to 8 so moreover PCIe multiplexer might become necessary).
This will change in next generations, it will be included.