Commerce & PayPal - What does "Solution Type" mean?
Ignore the terminology, it really makes no sense. What it should say is this...

Require customers to use a PayPal account?


[Sole] No, a PayPal account is optional. Customers can still choose to pay with a credit card instead.
[Mark] Yes, a PayPal account will be required to complete a purchase.




Here's a screenshot of the documentation...



So why did PayPal decide to call this particular choice "Solution Type"?

And what do "Sole" and "Mark" actually mean?

The world may never know.