Object Process Methodology prescribes more semantics to nodes (bubbles) and edges (lines). The project has largely been led by Dov Dori, from the Technion in Israel. I’ve seen the work at INCOSE (International Council on Systems Engineering) meetings, and have been impressed (i.e. it’s simpler than SysML, that was derived from UML). http://esml.iem.technion.ac.il/introduction-to-opm/
There is a Java-based software modelling tool available. http://esml.iem.technion.ac.il/opcat-installation/
While the research is conducted at a university, and OPCAT is licensed as GPL, I haven’t seen the project as open sourcing (i.e. sharing the source code in a public place, and welcoming contributors) in behaviour.
There seems to be a cloud version under development https://www.opcloud.tech/
In my conversations, I had mentioned that I use OPD (Object Process Diagram) conventions, i.e. boxes are object, ovals are processes. In prior uses of OPCAT (a few years back), there wasn’t a good export to SVG. Thus, I’ve been using graphical tools (e.g. Graphviz, draw.io) with aspirations to be like an OPD, but have been sloppy with the notation. (OPM Object Process Methodology draws the diagrams and writes code at the same time).