Payment processor
A payment processor is the third-party company that actually collects card details and moves the money on a platform's behalf. On adult platforms the checkout page is very often the processor's page rather than the site's, which is why the design, the domain and the company name can all change at the moment you pay.
Where you see it
At the point of purchase, in the address bar of the checkout page, in the sender name on the confirmation email, and again in the wording of the entry on your bank statement.
Why it matters
It explains why a statement line rarely matches the site you used, and it means cancellations, receipts and refunds sometimes run through the processor's own portal instead of platform support. A padlock on that checkout page only says the connection is encrypted; it says nothing about who is being paid, so the company name is worth reading before you continue.