Order of command block execution if triggered at the same time?

In Minecraft 1.9 command blocks get an overhaul.
There will be three modes of command blocks:

• Impulse: Regular behavior, command gets activated via redstone
• Repeat: The command in this command block gets activated every tick (superfast clock)
• Chain: This is the one you're looking for, see below

Command blocks also have a direction now, they can face north, south, east, west, up or down, this is important for the chain command blocks.

When any command block gets activated, it will activate the chain command block that it is pointing into. This means you can build chains out of command blocks, whose commands get activated in the order they are pointing into each other.

This is not a good idea. If you have commands that must be executed in a specific order, separate them with repeaters or comparators so that they execute in a specific order.

If you trigger a lot of command blocks with interfering actions at the same time, they will execute in a specific order, but you can't know what this is in advance, it may change without notice (if the server is restarted for instance), and will almost certainly cause you a lot of headaches.

It starts with the lowest z coordinate, then runs them in order of increasing x coordinate, then starts again with the next z coordinate. I do not know how the y coordinate is involved.