In Settings below, a cluster (e.g. cluster “root”) is set to recursive.

For the recursive case, use the terminal (or windows explorer/finder) to create subfolders in cluster “root”. To add cluster model“

>pwd
.../project
>ls
EIFGENs/     project.ecf  root/
> cd root
> mkdir model
> ls
application.e  model/

On compiling, the new cluster name model appears. Right click on model to add a new class in that cluster.