Solid-block vials are the most expensive of the three. Each maker sets its own standard for accuracy. You put a level on a reference surface, such as a big slab of machined granite, and then measure the amount you have to raise one end of the level to center the bubble. This number can also be described as an angle. The 0.001-in. angle is 0.059 .
Some manufacturers stamp the accuracy right on their levels.
I found that many manufacturers build their best levels to this 0.001-in. The next step up is a level good to 0.0005 in. Look at the math and decide for yourself. If you use a level built to the 0.0005-in. They assume you take your time sighting the bubble, and that you remember a level usually has one reference surface, not two.