''How one VBA project can use an object declared in the class module of another project''
*** http://support.microsoft.com/kb/555159

VisualBasicForApplications (VBA) enables one to give classes the "PublicNotCreatable" property to indicate that the class is "public" -- meaning usable by this and other projects, but instances can only be created by code within the current VisualBasicForApplications project.

(A stronger version of this is supported by a number of ObjectOriented languages:  By making the constructors private, one can insure that only code within a class can create instances of that class.)

This kind of feature is typically used in SingletonPattern implementations.