What does the thickness of CA actually mean?

CA Glues are generally broken down into Thin, Medium, and Thick or Gap Filling. The actual measure of “thickness” is Viscosity, which is measured (in the case of CA) in Centapoise. Centapoise is the ratio of force to flow at a molecular level, and is indexed at the viscosity of water. For our purposes however, it is the rate at which a glue will flow over a surface and be absorbed by surfaces. Mercury (and several other CA manufacturers) engage the Centapoise Value in the names attached to their various glues. So, Thin CA, M5T has a centapoise value of 5, while Thick M1100G has a centapoise value of 1,100. None of this helps much without a reference so this chart below, produced by V&P Scientific (http://www.vp-scientific.com/Viscosity_Tables.htm) may prove beneficial as you choose which glue to order.

Approximate Viscosities of Common Materials (At Room Temperature-70°F)
Material	Viscosity in Centipoise
Water	1 cps
Milk	3 cps
SAE 10 Motor Oil	85-140 cps
SAE 20 Motor Oil	140-420 cps
SAE 30 Motor Oil	420-650 cps
SAE 40 Motor Oil	650-900 cps
Castrol Oil	1,000 cps
Karo Syrup	5,000 cps
Honey	10,000 cps
Chocolate	25,000 cps
Ketchup	50,000 cps
Mustard	70,000 cps
Sour Cream	100,000 cps
Peanut Butter	250,000 cps

How Should CA be Stored?
CA likes it cool and dry. Store CA in the original container, and in the case of Mercury CA, be sure to remove any added tip and replace the cap as it is specially designed to reduce clogging. Do not freeze it or keep it near freezing in the refrigerator. The ideal temperature is 40 degrees Fahrenheit (approximately 4 degrees Centigrade) and 10% relative humidity or lower.
The good news with Mercury is that however you store it it is unconditionally guaranteed to not set in the bottle. If it does just return it to me and I will replace it with new.

Mercury Naming Convention
Mercury CA Adhesives are named in a particular way, which can tell you a great deal about the contents of the container. For example, M5T is a Mercury CA product Centapoise value 5 Thin, while M100F is Mercury Centapoise 100 Foam friendly, and M300FM is Mercury Centapoise 300 Flexible Medium.