Bly supports the visualization of files containing assembly strcture, where assembly is defined has an object (father) having other objects physically connected to each other (children) which all together form the final indipendent unit.
STEP files, as well as .bly files, may have an assembly structure in it: please note that this doesn't mean that they have relationship with external file, all the components are included into the father's file (see picture)
On the left you can see the assembly tree (see chapter Assembly Navigator), that shows the assembly structure and its children
