Why do some Helper Key Figures in UNIPA Planning Area have a Base Planning Level, and others not?

All of the following are Helper Key Figures. One can see that some of them have a Base Planning Level (column 3), but others not.

Why is this?

I see no option, when creating a new Helper Key Figure, to choose a Base Planning Level.

Further more, the IBP Model Configuration Guide mentions that Helper Key Figures "do not have a base planning level". Types of Key Figures | SAP Help Portal: