Juniper Part number confusion

So this is another one which should be very obvious but again I'd never questioned it until recently, so I'm jotting it down for reference.

For Juniper's larger routers (MX etc) there are a number of items which are redundant - PSU, RE, SCB etc. Now when looking through the price list these items each have 3 different part code options: -BB, -R, -S

Here are the differences:
-BB = Base Bundle - This item is purchased when buying a new chassis and is often discounted because of this. You cannot purchase this item with the complete chassis.

-R = Redundant - This item is purchased with a complete chassis when you want a redundant item. For example, if you require a chassis with 2 SCB then you would purchase 1 -BB and 1-R.

-S = Spare - This item is purchased often if you are upgrading an existing chassis - I.E. replacing an item in an existing chassis with a new item, or adding a redundant component to an existing chassis.