In the broadest of terms, no human may interact with a xenos creature in any way except violently—"such creatures are inherently blasphemous against the God- Emperor and must be eliminated on sight without exception". From a young age, citizens of the Imperium are taught to fear and hate the xenos, and innumerable legions of Imperial Guardsmen are taught of the superiority of Man over all inhuman things as they are drilled in the use of lasgun and bayonet. The number of xenos who have set foot upon human worlds peacefully and survived is small indeed.
But this is not always the case. Though the Imperium was founded upon a creed of intolerance towards non-humans, peaceful contact with a variety of xenos species "actually happens frequently". In most instances, these are short-term alliances or matters of temporary mutual convenience. Just as frequently, illegal contact with xenos cultures is sufficiently commonplace that the The Inquisition and the Adeptus Arbites keep a constant watch for potential contamination through contact with xenos; in spite of such vigilance, the “Faceless Trade” of xenos goods on the black market still exists. A number of xenos species hire themselves out as mercenaries to a number of other species, for reasons all their own, and there are more than a few "unscrupulous" Imperial Commanders willing to hire such creatures to bolster their forces, seeing them as the ultimate expendable mercenaries and caring little for their fate.
Rogue Traders, and a few other individuals, have the power to do more, and do so legitimately. These men and women, freed from the normal strictures and feudal requirements of the Imperium, have the influence and authority to make peaceful contact with xenos species for trade, temporary alliance or even to hire them as mercenaries. In some cases, an allied xenos may almost become a trusted companion, highly valued for its skills. Individuals with trusted xenos allies may find that the normal laws and doctrines against consorting with xenos hamper their operations, and in regions on the fringes of the Imperium, like the Askellon Sector, a method of sanctioning xenos allies has come about to ease these difficulties.These Sanctioned Xenos are still regarded with caution and hostility by most, but this is unsurprising—even the Sanctioned Psykers of the Imperium are still subject to the prejudice against "witches" that is so readily promoted in the Imperium. However, so long as they remain in the company of an ally who can vouch for their service (assorted documentation in copious amounts is provided for this purpose), and "agree" to be visibly marked to prove that they are Sanctioned—this mark is typically a brand or tattoo on the face or hands, where it can be clearly seen. The permanence of such a mark is of no concern—without a human employer to vouch for the xenos, the mark is essentially meaningless, and should the xenos cease to serve the Imperium, its life is forfeit anyway, and the creature is slain as soon as possible.
Within the Askellon Sector, obtaining such Sanctioning is an expensive and challenging matter; there are few who will even concede that such a practice exists, let alone perform it. A handful of Administratum Ordinates in Port Aquila, Port Lockhart and Terminus Prime provide the service. Usually, they have been recruited for such a purpose by interested Inquisitors, whose purity is tested on a regular basis to monitor potential contamination. Their identities are not widely known, and the service itself must be specifically requested, as they will not offer it otherwise.
As rare as Sanctioning is, it is recognised in many of the more well-travelled parts of the Sector and even some of the Low Worlds. As the required mark is very similar to that which identifies Sanctioned Psykers, it quickly becomes apparent what the mark means to those with any knowledge of the structure of the Imperium. It is still wise to be cautious in bringing even a Sanctioned xenos to a human world; those who will understand the nature of the mark and the notion of Sanctioning are still a minority, and a xenos creature "will spread only fear and anger" by its presence amongst the common masses of mankind.