Add a section
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Right-click between slides and select Add Section.
An Untitled Section is added in the thumbnail pane, and the Rename Section dialog box opens.
Type a name in the Section name box.
Select Rename.
To collapse a section, click the triangle next to the section name. The numeral by the section name shows the number of slides in that section.
Move or delete a section
Select View > Slide Sorter.
From here, you can:
Source: Microsoft