Using my Google-fu, I found some references.
Here's something from Adventure-7 Broadsword from classic Traveller:
Imperial Rules of War
To mitigate the potentially most disastrous aspects of armed conflict, the "rules of war" have evolved as an accumulation of unwritten concepts on a case-by-case basis. The rules of war have never been officially codified, both to prevent them being seen as an Imperial endorsement of war and to prevent formal precedent from preventing Imperial intervention whenever the Imperium deems it necessary. The main aim of the rules is to maintain the economic and military well-being of the realm, and the Imperium will intervene only when military action threatens this. The primary causes of instability, as viewed by the Imperium, are long-term economic dislocation and excessive extra-planetary influence.
Long-term social or economic dislocation is suffered when a region suffers some permanent or semi-permanent loss in its ability to carry on at its pre-war level of economic activity.
The excessive extra-planetary influence concept is even more vague. The Imperium tolerates the use of force as a necessary outlet for built-up political and social pressures. In such cases, a short war is deemed preferable to continuing tension, sabotage, political agitation, and so on. However, attempts by extra-planetary forces, such as off-world governments or large commercial interests, to seize control of a world's affairs are beyond the scope of the "safety valve" rationale. "Assistance" is tolerated, so long as it is deemed appropriate to the level of legitimate interest in the affairs of the world held by the extra-planetary organisation.
For example, the Imperium tolerates mercenary forces which have been hired by local governments or other on-planet powers. It has even tolerated the provision of training cadre, arms, equipment, and so on (on a limited scale) by megacorporations , and even of fully-equipped striker units to local forces. However, when it has appeared that the primary burden for the conduct of the war has been carried out by an extra-planetary power, the Imperium has intervened, claiming the power is using the misfortune of a local dispute as a pretext for aggression.
Unlike the above rules, one prohibition is clear and firm throughout the Imperium: use or possession of nuclear weapons, if discovered, and regardless of size or type, will almost certainly trigger Imperial intervention. The Imperium alone retains the rights to such weapons.
Here's a shorter, but essentially similar explanation:
The Laws And Customs Of War: The Imperial Rules of War, which are an unwritten guideline as to how Imperial vassals will settle difficulties between them. Basically they boil down to, "Have fun boys, but don't make to much of a mess because The Emperor has means to punish you."
The Imperium acts as Combat Referee between its substates. There are a number of guidelines to when an imperial intervention will be launched. War crimes, or the use of WMDs on the ground (in space it is different), being too obviously the aggressor, trying to grab too much power or to disturb too great an area and so on are likely to attract an Imperial intervention. The general rule though is "be unnoticeable and you won't be noticed".
Client planets are pretty much allowed free play so long as a war doesn't go on too long or cause too much death and destruction. There are a few hard and fast rules, however, which all local warmongers should bear in mind: Do not harm Imperial officials, soldiers or nobles; don't throw around nukes and other WMDs (the Imperium reserves that privilege to themselves); and don't ever interfere with trade.
And I see that GURPS: Star Mercs has additional info regarding this. But I don't have a digitized copy of it handy.
