The license key credits represent the number of pages the Design & Compose Server can generate per minute. So for example if your license has 500 credits, the server can create up to 500 pages per minute.
A credit is easy to understand and is independent of the complexity of document templates, document processes or hard- and software.
The following list all count for a single credit :
1 page in PDF, PDF/A
1 page in AFP, PCL, PS
1 page in TPCL, ZPL
1 page in DOCX, ODT
1 TIFF, JPEG, PNG, SVG file
1 XSL-FO file
1 HTML file
1 HTML Form request
1 schedule start
A page is always A4/letter sized and single faced. Recto-verso pages count for 2 credits. Larger pages count for multiple pages, for example A3 pages counts for 2 credits.
An HTML form is an enriched HTML page, designed to allow intelligent interaction with end users. Only the initial request for a session is counted.
If the requests you send would generate more pages then there are credits available, the page generation will take longer, until the credits become available again.