In PowerPoint, horizontal and vertical logic refers to two different ways of organizing and presenting information.
A horizontal storyline follows a linear progression, where each slide builds upon the previous one to tell a story or convey a message. You should be able to quickly flick through the presentation to get an idea of the story that the document is telling.
A vertical storyline is held on a single slide and builds up the action title, with each slide providing more depth and detail on that theme.