ECC; ECSC - What's the difference between Plant and Materials?

Serious question! My client wishes to compensate contractors for the current steep increases in Materials costs and has devised a Z Clause accordingly. But the Z clause only refers to Materials, not Plant. Where is the dividing line between Plant and Materials?
I guess that most would agree that a pump is Plant. But what about a steel staircase, or a trash screen? Are they Plant or Materials? Perhaps if they are manufactured off-site by a third party supplier they would be Plant. But if they are constructed on site, or even manufactured in the Contractor’s premises, would they now be materials?
Any thoughts?

There is no dividing line between Plant and Materials as the term “Plant and Materials” one define term and not two. The Z clause should be amended to refer to “Plant and Materials”. Rather confusingly the Contractor’s plant that is used to construct the works is defined as “Equipment”. So if the pump is a sump pump used to dewater an excavation during construction in the NEC that’s “Equipment” however if the pump is being installed into the works say as part of a heating system then it is “Plant and Materials”. Basically the Contractor takes the Equipment away at the end of the job but the Plant and Materials form part of the works and are left behind.

Thanks for the quick response