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.
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Type a name in the Section name box.
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Select Rename.
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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:
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Move a section: Right-click and select Move Section Up or Move Section Down.
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Delete a section: Right-click and select Remove Section.
Source: Microsoft