request
A request is one call to the service: you send what the document should say, and you get the document back.
In more detail
There is nothing to install and nothing running on your side. Your program sends a description of the document — the text, the tables, the pictures, where each of them goes — and the answer is the finished file.
The description is written in a plain form any language can produce. The same description sent twice gives the same document twice.
Unless the description asks for the moment the document is made, or locks it without stating the key it is locked under: those two differ each time.
What it does on a real document
Every example on this site can be asked for that way, and the third source file published beside each one is exactly that description.
One call means one PDF document, and PDF documents are the only thing your plan counts. A book of a hundred pages asked for in one go counts as one, exactly like a one-page invoice: the length of a document never costs you anything.
Where it comes from, and why
Sending a job to a machine and getting the result back is the oldest shape in computing. What is new is only that the machine belongs to somebody else and the job is described in plain text rather than in a program.
That is what lets a firm use an engine written in a language it does not use, on a system it does not run.