There is 3 types of Category.
Not Used : This Type is used to add Features Choices to the grid.
Manual Input : This Type is used when you need to enter a manual value. It can be used to enter a note or to enter a dimensionnal value
Sub Grid : The Sub-Grid type is used when you want to add a Sub-Grid to your grid. Adding a sub-Grid to your grid give the ability to have a Feature Grid common to more than 1 Feature Grid.