Anytime you want work in "interactive" mode (create, edit, render), the license is needed. Please, feel free to check this post
about "interactive" x "non-interactive" modes differences.
Our licensing system is very similar to the "3ds Max licensing system" - it's detecting when the scene is used in interactive mode (so the license is needed) and when not.
For example, when I´ll want to open any 3ds Max's scene just to render without any edition of included content, the 3ds Max will ask for the license to launch itself. There is exception while using network rendering so the 3ds Max session is not interactive and managed by the host. The same behaviour maintain our software.
Actually, take in mind the fact you can install & use our software in "Render slave node" on as many as render nodes wanted. So, please compare it to render engines licensing schemes (charging for every render node) and you'll see how generous in reality our licensing system is.