I noticed that a few basic guides are available here: https://www.archimatetool.com/developer
When googling around for something more specific, I came across this page: https://www.archimatetool.com/dev/model-new-element
This one is not listed under the developer resources, is there a table of content page somewhere that I'm currently overlooking?
I am also wondering if there is a way to tell if the guide is outdated. I tried modelling a new element. The guide and screenshot instruct to add a "BusinessLayerElement", this however is not listed when I try to do so. I added a "BusinessElement" which appeared to be the same. I noticed other differences as well between what was instructed and what was possible. Is that guide written for another version of Archi?
Yes, that page is well out of date and aimed at Archi 3. I'll remove it it until an update is available to save confusion.
The main problem with adding new concepts to the metamodel in this way is that it only works on that one version of Archi. You can't share a model with the new concepts unless they have that version of Archi, too.