The difference between structural unit and repeating unit?

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1. The problem statement, all variables and given/known data

The difference between structural unit and repeating unit

2. Relevant equations

3. The attempt at a solution

wikipedia gives an example with polyethylene terephthalate but I can't figure it out based on the slight difference
Polyethylene teraphthalate has two structural units but only has one repeating unit.

Structural units are substructures that are derived directly from the inputs.

If you call the repeating unit [-R-], then you can write a polymer as -[-R-]-n.

So while polyethylene teraphthalate has the repeating unit [-O-CH2-CH2-O-CO-Ph-CO-], it's built from two distinct inputs and each of those is a different structural unit.

To take another example, polyethylene, which once was also called polymethylene, -[-CH2-]-n, has the repeating unit [-CH2-] but has the structural unit [-CH2-CH2-] because it is derived from ethylene, not methyl radical.
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