Sections is a great feature to group together content in your newsletter! To add a section to your newsletter, go to Content > Sections. Once you create a section, you will see the module for it in your Design > Layout page. From there, you can choose how many articles are allowed to be displayed and where it is displayed within the template.
You can toggle on specific sources for a section so any articles we gather from those sources are appear together within the newsletter. For example, if my lifestyle newsletter has a cooking section and fashion section then I would toggle on sources related to cooking under the cooking sections and sources related to fashion under the fashion section.
You can also manually add articles to a section. For example, you may not have a feed for your upcoming events page, but you can use 'Add Article' and then assign a section to make sure that the article shows up within that section in the template.