The Portable Tank is, exactly as it sounds, a tank for the storage and transport of liquids. It works similar to a standard BuildCraft Tank but with a few very important distinctions. Each tank can only hold up to 8 buckets of liquid, opposed to the 16 offered by the BuildCraft equivalent. So they are not ideal for storing large quantities of liquid.
A default tank has two input faces (blue), one on the top and one on the bottom. However, the bottom face can be changed to an output (orange) by Right-clicking the tank with a Crescent Hammer or Prototype OmniWrench.
In addition, tanks will try to output their contents into any 'liquid inventories' connected directly to the output, i.e. other tanks or machines like the Liquid Transposer. Because of this, tanks can be stacked on top of one another, to form larger tanks, but only as long as the tanks are set to output from their bottom face.
This type of tank can also be filled and emptied using Buckets.
However, the most notable feature of the Portable Tank is that it can be picked up, by Shift Right-clicking on it with a Crescent Hammer or Prototype OmniWrench, and it will retain any liquid stored in it. The tank can then be placed again and the contents will still be there. While it is in your inventory, the contents of a tank cannot be accessed.