"Custom shows is all about creating a presentation within a presentation. Instead of creating multiple PowerPoint files, nearly identical presentations for different audiences, you can group together and name the slides that differ and then jump to these slides during your presentation. The slides in the show can be re-ordered to appear in a customized sequence.
For example - you might want to give a presentation to two groups that work at two different locations. The slide show includes slides 1 through 15, which are identical for both groups, and two custom shows, each specific to one of the locations. You can show the first 15 slides to both groups and then jump to a custom show named 'location 1' for the first group and to a custom show named 'location 2' for the second group."