request
A request is one call to the service: you send what the document should say, and you get the document back.
What the word means
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.
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.
What your plan counts is these calls and the pages they produced. A book of a hundred pages asked for in one go is one call and a hundred pages, and both figures appear on your account.
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, without installing anything at all.