The local outfit where I get my propane explained it this way - after I noticed it wasn't completely full according to the tank gauge:
"FULL" in a propane tank takes into account the expansion and internal pressure of the tank. Propane is stored as a "liquid propane gas" (LPG) which changes from a liquid to a gas before it is expelled under pressure from the tank. That conversion happens VERY quickly - as does expansion of the tank contents from external heat.
There is what's called the
"80% Rule" with propane. 20% of the tank MUST be available for expansion - both to allow for the conversion from liquid to gas, AND for expansion from external heating of the tank and contents. Thus, tanks should never be filled to more than 80% of capacity.
Anyone who fills beyond that 80% point is both ignorant of the extreme explosion risk, and doesn't respect common safety procedures.
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