Unpopulated root signatures.

Hi, (from: https://msdn.microsoft.com/en-us/library/windows/desktop/dn899109(v=vs.85).aspx#Null_descriptors) Does this mean that, I need to declare root signatures with all the slots even if I don't use them, or does it mean that every entry in the root signature must be populated? Sorry if I it is too obvious but I've read things related to this from different sources and I'm a bit confused :S. Thanks.