I’ve seen some of the topics in the forum talking about it but couldn’t find an appropriate answer (or let’s say one that I undestand ^^).
- Universal Descriptors : are used to create systematic descriptors for concepts
- Slots : are used to create systematic descriptors for concepts, and can be bulk added with ##automatically.add.template
In essence, my question is : when would you use one or the other preferably? What is the difference?
(I’ve been using remnote >2y and still ask myself basic questions 
)
EDIT:
Soren provided an appropriate answer 
Slots (as part of templates) are useful when you have a bunch of Concepts that follow the same pattern, and you want to ask the same questions about all of them. Your chemical elements are a good example here. Universal descriptors are useful for abstract concepts that cut across different types of knowledge (for instance, an ~Advantage would be relevant when talking about chemical elements as used in engineering, as well as political policies, productivity tools, and more).
It’s valid and even common to have a slot that is also a universal descriptor. For instance, if you’re studying medical treatments, they probably have ~Advantages, ~Disadvantages, ~Why Works, etc. – these universal descriptors could be used in totally different contexts as well.