[samus]

Internally, it uses Guile. I'm afraid there might not be a good user interface to edit the Guile source files, but at least Guile is much more pleasant to work with than C and offers way less paper cuts. I think it's mostly used for reports. You might have to create a plugin to expose your own scripts in the UI.

[kreyenborgi]

I feel like it could do with some tutorials and guides on Guile. I would have liked to be able to do reporting and invoicing with more control through scripts, but last time I looked at it I just gave up due to lack of docs.

[jrootabega]

Yeah, at some point I got the impression that I could use Guile to extend it, but then couldn't find any way to do it as an end user. Not sure if I was just wrong, or out of date.