To_Synonym
Analogous to to_name, a relationship can use to_synonym to indicate which synonym the class in the to property takes as role in this relationship.
Like other synonyms, this gives the AI agent your terminology for the class in this role, improving answer accuracy and autocomplete.
Example
A Flight departs from an Airport, which would be called in this case the Origin Airport. We may want also to add a to_synonym for it as Departing Airport.
relationship departs_from {
display_name: "departs from"
to: kb.Airport
to_name: "Origin Airport"
to_synonym: "Departing Airport"
sql: ...
}