What does the "Leg" stand for in reference to the "Agent Leg"

The term is not coined by RingCentral. It's been in the telephony field for ages. Google "call leg" or "agent leg" and look at images. Also look up "call structure"

I am copy pasting some info for you in the context of Contact Center.
Agent Leg = Connects the Contact Center to the correct RingCentral device, so that Contact Center call can be interacted with.
Contact Center = Central point where all customer interactions are managed across various channels, where contacts are not assigned to a specific agent, but rather a skill, which is handled by a team of agents.

By the way I don't have a deep understanding of this, so this is the best I can explain it. Don't think it's that important?