If you step away from your ruck and come back to find someone else sitting on your ruck, you might go into “ruck rage.”
“Ruck rage” will manifest itself in a sudden and complete change in demeanor as you quickly and violently question the offender as to why they are sitting on your ruck. Expletives are common. Offenders will usually vacate the ruck immediately, while some will advise the “ruck rager” to “calm the fuck down, bro.”
One’s ruck becomes his/her home when in the field. And while you, the owner, are allowed to treat said ruck in any way you wish so long as it remains serviceable, others are not afforded that privilege.
Best practice: sit on your own ruck or sit on the ground.