Location: "Imperial Society" is the society of the nobility, and is practiced wherever they congregate.
For example: sector capital worlds, the upper spires of most hives, portions of pleasure worlds, the capitals of prosperous agri-worlds, and so forth. Keep in mind, however, that the more distant from the centers of power and fashion, the more out-of-date and provincial the version of Imperial culture practiced.
Holy Terra might be said to be dominated by the Imperial culture, but in truth Terra is as much influenced by the distinct subculture that holds sway within the ecclesiarchy. Trade ships and space stations have a common culture within a given sector, but it is not Imperial; it is, rather, the distinct culture of spacers, influenced by the Machine Cult (as they depend on machines constantly) and rife with strange superstitions born of proximity to both void and warp. Stations also likely contain enclaves of the cultures of nearby planets, made up of those hardy souls who would risk a life in space for the hope of profit and a chance to slip the bonds of the life they were born into.
Descent: Strictly speaking, one need not inherit a place in Imperial Society. However, a lack of illustrious ancestry is one of many things that will reduce your status in the eyes of your peers and betters.
Imperial Society is obsessed with status; sector nobility (those whose influence touches several worlds) look down upon planetary nobility (those whose power is confined to a single world,) who in turn look down upon regional nobility (those who govern a single city or portion of a hive, controlling the lives of a paltry few million souls.)
The origin of one's influence is equally important; families whose power comes from longstanding ties to the church disdain those merchant princes whose power comes from the trade and transport of mere material goods. Families with a history of service as high-ranking officers in the Imperial Guard sneer behind their hands at the priests, and so on.
Therefore, if you wish to be fully accepted into Imperial Society, you must either have personal achievements that outweigh your low birth (as any formal party is enlivened by the invitation of a genuine hero) or marry into an established family; preferably both.
Language: The languages of Imperial Society are High and Low Gothic. Formal events are conducted in High Gothic. Normal conversation is conducted in a mixture of the two; primarily Low Gothic, with frequent quotations or borrowed words from High Gothic. This is done to show the speaker is cultured, educated, and generally better than you, like someone with a classical education who constantly quotes in Latin.
Individual families, however, may use a different language for internal matters. For example, House Cassius, a wide-spread merchant family, uses Stone Tongue, the dialect of their homeworld Plutarch's Rock, in private conversation as a simple precaution against being casually overheard.
Customs and Culture: The heartbeat of Imperial Culture is its parties. Here, over fine amasec and off-planet delicacies, status is evaluated and re-evaluated, information is disseminated by the medium of rumor, one can do a headcount to determine who has killed who in duels, and, very importantly, you can see what people are wearing.
Personal ornamentation is very important, not merely as plumage but as communication. A discrete symbol shows that your family takes pride in its piety and enjoys the power that comes from an uncle being a cardinal. A military dress uniform implies that you are a war hero just stopping by for a chat and a drink before rushing off to save the Imperium. An oddly curved earring informs those in the crowd who know what to look for that you are a cultist of Slaanesh, and might be of assistance should they find the current party boring. Two augmetic fingers advertise that you are Commisar Cain, and any noble ladies who wish to pull you into an alcove for a private chat will not be dissappointed.
To be accepted into Imperial Culture, one would need fine clothes, the ability to speak both High and Low Gothic without an embarrassing accent, a working knowledge of etiquette so you don't shame yourself by choosing the wrong fork or telling a bawdy joke to the baroness, and an introduction by someone already accepted by the culture.
Beliefs: All in the Imperial Society follow the Imperial Creed, at least in public. It would be social suicide not to. In private, they run the gamut from the truly, deeply pious, through the spiritually apathetic, to those touched by chaos. The worship of Nurgle is rare among the nobility. Khorne has a following among some families of a military or murderous bent. Slaanesh and Tzeench are quite popular, among the decadent and ambitious respectively.
It should be noted that among vamilies engaged in bloody vendetta, "having a spiritual awakening" and becoming a monk or nun is an acceptable way to escape the cycle of assassinations by abandoning all claims to power.
Material Culture: Fine clothing is a must. Xenomesh armor may be worn to protect against assassins, as it is far less crudely obvious than other armors. Grooming is of paramount importance; beards must be neatly trimmed, hair well coiffed. For personal protection, a high-quality las-pistol or two may be carried (I favor Palantines;) stubbers are sometimes regarded as crude tools of the violent lower class, and carrying a bolt-pistol openly announces your intent to cause mayhem. If you don't have a personal servant or personal servitor, for heaven's sake don't let anyone know. A small amount of fine jewelry is encouraged as a discrete display of wealth.