From IFWiki

Southeast (or south-east) is a direction. It is one of the four diagonal directions, and one of the eight compass directions. Its opposite direction is northwest.

Southeast is associated with the bottom right corner of a map; most maps are meant to be read with the north-end up. If you are facing north, then southeast is in a diagonal direction both back behind you and to your right.

Some early IF games do not support diagonal directions like southeast. In ADRIFT Generator 4.00, diagonal directions are initially disabled and must be explicitly enabled by an ADRIFT author before they can be used in a game.

The SOUTHEAST command

Typing SOUTHEAST means GO SOUTHEAST or WALK SOUTHEAST. That is, direct the player character southeastward from their current location. Assuming there's an exit in that direction and nothing is blocking or hindering the character, the PC will travel southeast to an adjacent location.

SOUTHEAST is such a common enough command that it has an abbreviation: SE.

Southeast in other languages

Some languages and regions prefer to hyphenate the names of diagonal directions. Some language library extensions support both spellings; others don't. Only the unhyphenated variants are listed here.

Dutch zuidoost zo
English southeast se
Esperanto sudorienten so, sor1
French sudest se
German Südosten so
Italian sudest se
Spanish sudeste se
Swedish sydost so

1 Esperanto's words for southwest and southeast are sudokcidento and sudoriento, which means it's problematic to use so as an abbreviation for either of them. There aren't enough Esperanto IF games to see any clear concensus on how to abbreviate sudorienten, but so and sor seem the most likely possibilities.

Code Compare

Directions like SOUTHEAST tend to be implemented both as room properties and as objects in their own right. The SOUTHEAST command, however, tends to be parsed or reinterpreted as a GO command acting on the southeast object.

Code Compare: Southeast
ADRIFT 4:Move SouthEast to Field begin.pngBathroomField end.png
Inform 6:The se_to property and se_obj object
Inform 7:The southeast direction
TADS 3:The southeast link property and southeastDirection object