From Technic Wiki
The Ender Tank offers stationary storage for up to 16 buckets of a single liquid. Like the Ender Chest, it has three wool pads on top which can be manipulated by right-clicking each one with a dye. Tanks with matching colors will share liquids between them, even across dimensions. Additionally, right-clicking the dial with a diamond will set that tank to a 'private' network for that player, change the dial to a diamond color, and show your username in the tooltip. A private Ender Tank will not share any liquids with any public tank, or any private tank from another user, even if the colored pads match.
Note that while using an Ender Tank with no diamond, without additional security plugins or mods, there is nothing stopping someone else on a server from copying your tank's colors and taking your stuff. If you don't have diamonds to spare, the best thing to do would be to pick an obscure combination of colors and hide it out of sight, so nobody can see the colors. With three pads there are 4096 different possible combinations of the 16 dyes, making it very unlikely - but still possible - for someone to randomly guess the right colors.
One example of use would be an easy way to send Lava from the Nether to the Overworld, for use in Magmatic Engines. Tanks with different colors will not share liquids, allowing for many different networks.
On the front of the tank is a small dial with an orange and a blue side. Right-clicking the dial will spin it, toggling between orange (output) and blue (input). When the dial is set to blue 'up' (input), the tank will accept liquid being pumped into it AND liquid can still be pumped out by a Wooden Waterproof Pipe or an Emerald Waterproof Pipe along with an engine. However, when orange is 'up' (output), liquid will output all on its own into any connected waterproof pipe BUT no liquid can be pumped in. Upon receiving a Redstone signal, the dial on the tank will flip; upon losing that signal, the dial will flip back.