A BusinessRuleException is a subset of all exceptions. UiPath sends errors in the Catch portion of the Try/Catch to whichever is most specific and applies to the error thrown. Therefore, only businessruleexception exceptions will reach the BusinessRuleException part of the catch block, and all others would go to the Exception part of the catch block.
Not sure in this case. It depends on whether your try/catch for the business exception is in the catch block or the try block. It sounds like it would be better to have two separate catch blocks in a single try/catch block, however.
I know this is strange but Business Exception is the children of System exception. If you have a try and catch with a system.exception and businessRuleException and a business exception is occurring, the body of Business Rule Exception will be executed (the children).