Ply rating versus actual plies
Historically, ply rating related to the strength of cotton-cord tyre construction. Modern tyres use different materials, so 10PR is a strength/load-class description rather than a literal ply count.
Where you see it
PR markings are common on light-truck, commercial and some off-road tyre applications.
Do not shop by PR alone
The tyre still needs the correct size, load index/load range and intended application. A higher PR marking is not automatically “better” for every vehicle; it can also change weight, stiffness and ride behaviour.
For commercial use
Send the complete sidewall specification and vehicle/application so the correct load requirement can be checked.