Hi,
This is a very broad question. It mainly depends on what you are truing to do and based on that you should take a call which object would suit the needs best.
Some components are server side and some are client side. Client side actions work on the form mainly. Its also important to understand the order in which these things are executed.
If you have to change the field behavior - ui policy with ui policy action is the best choice
Some sort of Validation, client script (or sometimes ui policy) are the best options.
Business rules and Script include are server side objects , if you need to trigger an event or update any other record the BRs are the best place to implement it.
If there is a function that needs to be called from multiple places then Script include comes into play. It can even be called form Client side for a client script.
This is not a full answer but will give you an idea. Its best to study them on SN docs and when you understand them, you will be able to answer this question.
-Anurag
-Anurag
