//one

The //one type accepts only defined, single values. It rejects no-values (//nil) and collections of values.

This is equivalent to:

A longer way to write //one

{
  "type": "//any",
  "of": [ "//bool", "//num", "//str" ]
}

//one does not take any parameters.

/.meta/one, the schema for //one definitions:

{
  "type": "//rec",
  "required": {
    "type": { "type": "//str", "value": "//one" }
  }
}