Early versions of PERT diagrams had "PERT Events" that were **_all_** milestones or gates with task information embedded in the arrows. This has changed over the years but most PERT charts I have seen "in the wild" continue to show milestones.
Outline tasks or phases are rarely shown on PERT charts or other network diagrams, in my experience since they generally interfere with reading the diagram without providing useful information.
I could not find much academic/formal guides for this kind of diagramming. Lots of information on building PERT estimates but my Google-fu is coming up short on the diagramming. Considering how old PERT diagrams and methods are, I find that surprising.