When an engine is built, the machine is
graded in the unit: "Efficiency Cost".

Efficiency Cost is defined as:

"The amount of energy an engine USES,
to perform the work it was built to do."

Example: If an engine processes 98% of the
energy to perform the work it was built to do, it's
Efficiency Cost is 2% or "2 points", or "2".

Therefore

An engine with an Efficiency Cost grade of '2',
is understood to be 98% Efficient.