Now that I've had a little time to think about my 'shoot-from-the-hip' answer, let me add another example.
We also test some of our devices for air flow by attaching them to a test fixture with a flow meter. This is test hardware that must be calibrated at pre-determined intervals. The same reasoning would apply if you were checking any type of liquid or current flow.
I believe another type of fixture would be the type that physically tells you the part is good, e.g.; it slides under a bar, fits between pins, falls between the marks on a go/no-go gauge, etc. How does an operator know that the gauge isn't worn or slightly bent without a regular (documented) inspection of the gauge itself? Just be sure to tailor the effort involved to the nature of the measurement. No sense spending a lot of time and money if the spec being measured isn't critical to the function, but that's a company decision.
Hope this is of some use.
[This message has been edited by Alf Gulford (edited 09 October 2000).]