You're drinking milk right from the jug. A little dribbles down your cheek and lands on the counter. No big deal. One sheet off the paper towel roll and no one will ever know you're kind of a gross slob. Call that a tier 1 milk spill.
Now carry that theory over to oil spills. There are three categories of damage, as designated by the International Petroleum Industry Environmental Conservation Association, or IPIECA. Tier 1 is the least worrisome...the equivalent of the dribble off the chin. Usually, it designates a spill within the oil producer's own facility. A short-term problem with limited implications.
Next up, you've got the tier 2 spill. That's like if you sneezed while gulping the milk and blew a mouthful all over the cabinets and the front of your jammies. In the oil world, it's a spill that's big enough to require some help from authorities to clean up.
Damage has spread beyond the oil producer's facility into the outlying area. So basically, you might need your roommate to come in and help you mop up.
Finally, you have the tier 3 spill. The milk container drops out of your hand, bangs onto the counter (blasting liquid everywhere), and then empties out all over the dog, who panics and runs around the house, leaving little milk footprints everywhere, before finally shaking off in a little white explosion right by your roommate's laundry basket. Utter disaster. You consider just leaving a note of apology and moving out before anyone gets home.
In the oil world, this is the worst level of spill. It might involve a global response. A tanker bursts open in a protected ocean reserve. An offshore oil rig explodes and starts chugging crude into shipping channels and (with the help of the tide) across hundreds of miles of inhabited coastline.
