AllowAbstract

As a general matter, Abstract classes cannot be picked from the Editor class picker. Adding the AllowAbstract specifier to a Subclass or SoftClass property, and setting its value to true, will allow Abstract classes to be set for that property.

Consider the following abstract class.

UCLASS(Abstract)
class USomeAbstractClass : public UObject
{
    ...
}

Normally, you would not be able to select this class in a class picker dropdown. However, by adding the AllowAbstract specifier to the following properties, the abstract class can be selected.

UPROPERTY(Meta=(AllowAbstract="true"))
TSubclassOf<UObject> Subclass;

UPROPERTY(Meta=(AllowAbstract="true"))
TArray<FSoftClassPath> SoftClassPaths;