Object fields vs slots

[Phil Longstaff]

What is the rationale uses to decide whether to make an object attribute 
a field in a structure vs a slot?  An Account has the hidden, 
tax-related and placeholder attributes which are slots rather than 
fields in the Account structure.  What is the reason for that decision?  
I know slots are used to hold sx information.  Are they used extensively 
elsewhere in the system?

Phil