Up is awkward to represent on a two-dimensional map that already associates the top edge of the map with north. Vertical pathways may be drawn differently than level pathways, and often have to be explicitly labeled as upward or downward paths.
The UP command
Typing UP means GO UP or WALK UP. That is, direct the player character upward from their current location. Since most PCs are human and can't fly, this usually means that the character should implicitly use whatever nearby thing would normally help them ascend, so UP might mean CLIMB STAIRS when there's a stairway, or CLIMB TREE when the PC is under a tree, or CLIMB CLIFF when at the base of a cliff, and so on.
(Ideally, both the UP and CLIMB commands ought to work when both of them make sense, but it's up to the author to define both. Far more likely, only one command or the other is implemented per instance, and it's usually only UP that works -- unless it's meant as a puzzle, in which case only CLIMB might work.)
UP may even work on the very simplest of elevators, where the elevator itself isn't really implemented at all; it's assumed you just take the elevator up to the next floor. (When the elevator is implemented, a mere UP typically won't work, but you should be aware that both versions of elevators are possible.)
Of course, some player characters like angels, birds, vampires, and superheroes can fly; in those those cases, UP means FLY UP.
With the understanding of all of the above, assuming there's an upward exit and nothing is blocking or hindering the character, the PC will travel upward to an adjacent location.
UP may be redirected into another directional command or vice-versa. For example, if there's a stairway going up at the north end of a hallway, either NORTH or UP ought to do same thing: move the PC up the stairway.
UP may be ambiguous in some contexts if there's more than one method to go up. If it makes a difference whether the PC climbs the ladder or the ivy to get to the roof, the game should generate a disambiguation question, asking if the player means to CLIMB THE LADDER or CLIMB THE IVY.
UP may be initially disabled in some contexts if the method of ascension is considered a puzzle that must be solved first. Perhaps it's not obvious that the ivy is climbable, and the author requires the player to explicitly try CLIMB IVY to demonstrate that the player thought to test it. And maybe the ivy is only climbable when the player is wearing some gardening gloves. However, after the ivy has been proven to be climbable, a simple UP ought to work there, with the game automatically having the PC don the gloves if they weren't already on.
Up in other languages
Directions like UP tend to be implemented both as room properties and as objects in their own right. The UP command, however, tends to be parsed or reinterpreted as a GO command acting on the "up" object.
|Code Compare: Up|
|ADRIFT 4:||Move Up to Attic|
|Inform 6:||The u_to property and u_obj object|
|Inform 7:||The up direction|
|TADS 3:||The up link property, the upDirection object,|
and the defaultCeiling and defaultSky objects