OpenFoam classes vs libraries
 

Greetings for All:
How could someone recognize the library from the class?
It is known that they both have declaration and implementation files , we could recognize class from the library via preceding the class by the word class in its declaration file whereas it is not the case in the library and finally the way of their declaration and compilation is different. However, if it is clear about the need for some classes to be created ( or derived from other base classes) for performing some tasks which are not available in the OpenFoam, it is not clear about the library. What are the general features ( attributes) which make the library a library ? What are the circumstances we need to initiate a new library ? Is it enough to prepare a bunch of classes to achieve the same goal for which a library is created?
I understand that there is a good information available about the OpenFOAM libraries in the link:-
https://www.openfoam.com/documentati...p#x34-137001r3

But it is not enough to answer the above questions :)
Thanks for any idea or comment!
My best.