Sorry for the poor title, I'm asking this question because I can't think of a good way to phrase this...
When we describe the stress required to extend a spring to a given length, we describe this stress in Newtons - a single, specific quantity of force.
When we describe the stress required to extend a material to a given length, we describe this stress in Pascals (pressure) - an amount of force per area.
This is because a material is an abstract concept without dimensions, unlike a self-contained device with specific dimensions and boundaries. A material can have physical properties, but a device can be represented by a CAD model while a material cannot (only a piece of a material, which is itself like a device).
How can this distinction - these two categories of things - be described?
If that didn't make sense, here's another example.
A molecule can be thought of as a molecule if it has determined limits - it has a set number of atoms of each type. (Please don't get hung up on the specifics of why this definition is right or wrong, as that's not relevant to the full question).
A lattice cannot be thought of as a molecule because it is of indeterminate size - it does not have the same limits. It can stretch on indefinitely and when its behaviour is described it is thought of as doing so.
Again the same distinction is seen between the self-contained and the indeterminate. How do I describe this?