We see a few different use cases for the separate plan tab.
Using the separate tab for a mini separate project.
e.g.an Onboarding Plan, or a POC/Pilot plan.
"Light" next steps under the overview and separate the more detailed, long-term milestones under a different section.
The Overview kept clean for general users who are looking to get a quick understanding of their solution while isolating the mutual-action-plan to users who are more in the details like a project manager or a client champion.
There is no right or wrong, it depends on the situation. For the most part we recommend keeping everything under the Overview section, unless you feel that it might be overwhelming to certain users or if the Overview section is getting too long.