https://docs.unrealengine.com/latest/INT/Programming/UnrealArchitecture/Reference/Functions/Specifiers/Reliable/index.html
https://docs.unrealengine.com/latest/INT/Programming/UnrealArchitecture/Reference/Functions/Specifiers/Unreliable/index.html
These two specifiers both say “Only valid when used in conjunction with Client or Server.” but it is an error to use the specifier NetMulticast without one of them.
Also it might be helpful to add to the documentation for Server, Client, and NetMulticast that they need to used with either Reliable or Unreliable.