An eidolon is the common term for a sophont's telepresence in utility fog. Though there are a few clades and cultures that avoid, forbid, or restrict them, or for one reason or another do not employ the underlying utility fog technologies, use of an eidolon for communication is an extremely widespread phenomenon. An eidolon is generally seen as less personal than a visit in person, but more so than a virchspace interaction and far more so than text, audio, or audiovisual communications. It is commonly used as an alternative to a personal visit when direct contact is impossible because of great distance, radically different environmental requirements, distrust between the parties concerned, or matters of social convention and propriety. Some individuals who avoid using virchspace for practical or personal reasons will still consent to send or interact with an eidolon as an alternative.
Given the capabilities of standard utility fog, it is relatively easy to produce an eidolon that seems "real" by visible light, and makes sounds in the typical Terragen hearing range. An eidolon that is realistic as perceived by other senses (touch, smell, sonar, or electrical fields) requires specialized utility fog, and sometimes a much more sophisticated program. This can be a practical difficulty in interactions between radically differing clades, especially if they use different utility fog standards. The most elaborate varieties of eidolon may require the incorporation of specialized materials or technologies, and for this reason it is sometimes the practice to meet on purpose-built eidolon "stages".
Of course the "proper" use and design of an eidolon varies strongly from place to place, according to local etiquette, philosophy, and aesthetics. In some cultures it is rude, or even blasphemous or illegal, for one's eidolon to be anything other than a close representation of one's own physical state. Other cultures, by contrast, seek or even require variety in representations; one might have a different eidolon to present according to the circumstances of the visit, the number and social status of observers, the time of day, and so on. In such cultures the choice of the wrong eidolon can be the destruction of a personal reputation, and an event remembered for decades in the local gossip, whereas the choice of just the right eidolon for each occasion is a mark of high status and sophistication. In yet other cultures nearly all interactions are by a non-representative eidolon, and revealing one's own form (either by eidolon or in person) is a sign of great personal intimacy, comparable to nudity in many of Old Earth's human cultures. In some places an eidolon that is difficult to distinguish from the "real thing" is expected, and one may gain high social status by employing one, whereas in others, it is mandatory to use an eidolon that is obviously not alive, or is tagged in some way as a representative. Failure to abide by local law and custom regarding eidolon use is the mark of an inexperienced or inconsiderate traveler, and results in the usual penalties for cultural ignorance: anything from the snubs, mockery, and fines, in Sephirotic and other regions of the Civilized Galaxy through to imprisonment, death, or torture.
AIs often use a utility fog eidolon to interact with vecs, cyborgs, and bionts, particularly if, like the inhabitants of Nimbus, their computronium substrate is itself made of utility fog. A transapient's eidolon (sometimes called a "face" or "aspect") may be several different bodies, or perhaps an entire environment that hosts both real and utility fog bodies of subsingularity beings.