The maximum working load is the maximum load a chain can support before it will experience fatigue failure. The maximum working load is determined by carrying out fatigue tests in laboratory conditions but can be lower at certain speeds and temperatures and should be discussed with the manufacturer.
The measuring load is a proof load that should be applied when measuring chain elongation with a chain wear gauge. It is usually 1% of the chain tensile strength.
Multiplex chain consists of several strands of chain joined together by a common pin at every pitch.