The shell sets the airflow, the draught line and how much warmth the rest of the system can hold.
Chosen for the design night with a margin, and stored loose rather than crushed.
The ground steals heat all night. The mat is the part that loses the argument least often.
The neck is the fastest way to wake grumpy in a warm system.
The kit you change into at the tent door. Dry, warm and never worn hiking.
The small items that turn the system on and off the same way every night.