What role did the Yam play in the expansion of the Mongol Empire? It was a postal service that doubled as a spy network.
It was a written system of laws that kept peace and order.
It was a source of loans for merchants to promote business.
It was a period of peace when people felt free to trade and travel.

Yam (Mongolian: Өртөө, Örtöö, checkpoint) was a postal system or supply point route messenger system extensively used and expanded by Ögedei Khan and also used by subsequent Great Khans and Khans. Relay stations provided food, shelter and spare horses for Mongol army messengers.

Khan and known as the Yasa, which served to regulate personal status and criminal liability among the Mongol elite, as did the Sharīʿah among Muslims.

I hoped this helps!!