Because MXP uses the characters:
< , > , and &
as part of its markup scheme it is essential that MUD servers replace those three characters in particular with their equivalent "entity names", namely:
< , > , and &
You can also use any arbitrary ASCII code combination like this: á
The list of predefined entity names is at MXP Entities.
Defining your own entities
You can also define your own entities, like this:
<!ENTITY clan_name "Blood Clan" >
Then you could use this entity in MUD output like this:
You are a member of &clan_name;