The above is the Chassis.
In the back you have room for power supplies.
If your AC is 110V. This is called LOW
You can use.
2 Power
4 Power 2+2 for redundancy.
If your AC is 220V. This is called HIGH
You can use
1 Power
2 Power 1+1 for redundancy.
You can run
1 Routing Engine
3 Cards
or
2 Routing engines
2 Cards.
Parts with BASE/Premium3- have the old Chassis with older midplane connector.
Parts with BASE3/Premium3- have the NEW Chassis with the new midplane connector {used for SCBE2 and high throughput cards}.
Prem3 are built to stock parts.