Layout posts are a means to apply global content, content that must appear on more than one page of your site.
Regions
When you navigate to the Layout posts screen accessed from Layouts in the admin sidebar, you'll be presented with a list of regions. The layout assigned to a region will appear in just that region. For example a layout post assigned to "Below Content" will appear directly below the currently viewed page's content area.
Although you can add as many layouts as you like to each region, on any given page only one layout will be selected from each region. The layout that will be chosen is determined by the Visibility Conditions.
V4 will start at the very first layout in each region and test the Visibility Conditions on that layout. If they pass for the currently viewed page, it will be used. All layouts that are ordered after that layout in the region list will be ignored.
For example imagine you would like to have a homepage header and a childpage header. You could add the following condition to the first layout in the Header region list:
Your second layout, the childpage header layout, does not need any visibility conditions at all.
When someone is viewing the site's homepage, V4 will look through the layouts starting from the top:
The homepage layout has the conditions above, it passes, it will get used. Your second childpage header layout will be ignored.
When someone is viewing a childpage on the site, it will look at the homepage layout, see that the Visibility Conditions fail, so it does not get used. It will now look at the second layout which has no Visibility Conditions, and with no conditions preventing it from being displayed, it will be used.