Are transmission tags essential to control joints of a robot?

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I'd initially thought that every joint which needs to be actuated needs to be accompanied by a transmission tag in the urdf and needed to be interfaced with gazebo through a controller plugin But I came across fetch robot ( https://github.com/fetchrobotics/fetc... ) which works completely fine without any sign of transmission tags anywhere. I'm trying to emulate this setup exactly for my robot (a combination of Pioneer3DX and a 7DOF arm) but it's failing. What am I missing? How does the fetch robot control joints without transmission tags?