First and foremost, when referring to the "chassis" it's almost always going to imply the physical chassis, rather than the configuration statements from your question.
That being said, the chassis configuration statements are settings that directly relate to the chassis and the installed components within (FPC, SCB, RE, Craft Interface, etc.)
Here are a couple of examples that will hopefully provide some additional context.
- Network Services
Network services configuration has to do with how certain hardware-related features are either enabled/disabled or scaled. This can also determine if certain types of physical hardware components (typically line cards) are enabled/disabled as well.
root@vmx6# set chassis network-services ?
enhanced-ethernet Enhanced ethernet network services
enhanced-ip Enhanced IP network services
ethernet Ethernet network services
ip IP network services
- Routing Engine Redundancy
Routing engine redundancy settings allow you to determine what happens when the hard drive on a routing engine experiences an issue. With these, you can tell a routing engine to shut off entirely or perhaps just reboot.
root@vmx6# set chassis redundancy failover ?
+ apply-groups Groups from which to inherit configuration data
+ apply-groups-except Don't inherit configuration data from these groups
disk-read-threshold Read threshold (ms) on disk underperform monitoring (1000..10000)
disk-write-threshold Write threshold (ms) on disk underperform monitoring (1000..10000)
not-on-disk-underperform Prevent gstatd from initiating failovers in response to slow disks
on-disk-failure Failover on disk failure
on-loss-of-keepalives Failover on loss of keepalives