Why does solid copper matter for PoE?

PoE sends DC current through the same balanced pairs used for data, so conductor resistance and the resulting heat matter as well as the data specification. Solid copper conductors provide predictable electrical performance for the fixed cabling system.

That is one reason not to confuse these products with copper-clad aluminium cable. Similar-looking boxes can conceal very different conductors, and electricity remains stubbornly unimpressed by marketing adjectives.