These tags define the image layout in terms of how many horizontal segments can be logically formed within the layout. This is not necessarily how many rows it has, but rather how many row-like segments there are.
Base forms
The most basic structure the image layout can form described as the number of row-like segments within the layout. The higher the number, the more complex the layout tends to be.
Strict row layouts
These tags maintain a strict definition of what is defined as a row, disallowing rows to be subdivided into column segments. These are subset concepts of their respective base layout tag.