Universes
Almost everything that happens within the setting takes place within a universe. A universe is all time and space, including its contents in all dimensions of time, space, probability, possibility and potential.
Herewhere (Universe Two)
The setting itself, Universe One is the location in which most events unfold. It is our primary setting. It is sometimes called the hereverse.
Elsewhere (Universe One)
Originally only speculated, Universe One is a universe entirely outside of our own. It has entirely different physical properties, information, knowledge and events totally unlinked to Universe Two. Objects and events of Universe One are inherently comprehensible within Universe Two, making all attempts to investigate Universe One near impossible. Energy is known to disappear from the Herewhere into the Elsewhere spontaneously for reasons unknown.
Neverwhere (Universe Zero)
Briefly investigated, Zero was a failed universe which at some point in its lifespan was destroyed cataclysmically. The nature of this destruction is thought to be artificial and some information from the neverwhere leaks into the herewhere through weak spots in spacetime.
It is known that most forms of hyperspace involve dipping into Neverwhere partially, as it presents less friction and allows for faster than light travel by in theory, not actually moving faster than light, just moving the space around a craft faster than light.
The Neverwhere is known to have very strange interactions with frequencies of electromagnetism interactive with brainwaves, similar to psionics. In addition, the rules of cause and effect are known to loosen substantially with longer visits. Those who have been stranded in the Neverwhere seriously recommend against extended visits.
Those who do so intentionally are known as 'divers'.
Navigation
Objects have been known to move from either universe, though they incur an information hazard: that some objects - a kind of trans-universe petrification where equipment may stop existing, information may disappear from the memory of people and 'nearest equivalents' may be selected to fit into the universe in a process called transplantation.
This is caused by information anchorage: the notion that some forms of information (organized matter and data) cannot cross universes. The process may be immediate or gradual and is linked with the rate of change it can cause in the opposite universe (generally how potentially destructive or changing something is).
Crossing one universe to the other is generally a conversion process with partial information loss on some level and equivalent exchange in order to prevent paradox.
Information
Not to be confused with information as data, information states variables or patterns within a given universe – the facts, truths, names, peoples, events and places of history itself. This is all a pattern, referred to as information.
When crossing from some universes to other universes, knowledge of some information doesn't survive the trip and is no longer known by characters. This is referred to as expunged information. In many cases, those who travel have no idea they have actually left somewhere, or have made preparations to part, or that they have arrived.