Two or more nouns are sometimes used consecutively, with the second modifying the first.
For instance, I recently received a mail whose subject was "Honorarios migración." This is, I suppose, shorthand for "Honorarios de migración" or, if we're feeling wordy, "Honorarios para el servicio de realizar trámites de migración".
Other examples: at a parking garage: "salida coches."; spam from an airline: "¡Venta fin de semana!"; signs in my colonia that read "venta garage."
Some of these usages maybe be due to space constraints, but that clearly isn't the whole story.
So, first, what is the name of this phenomenon?
Second, is this something you'd ever want to do in formal writing? If so, under what circumstances?