2.2.1.4 https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_udf/flu_udf_GeneralSolverDEFINE.html
A UDS is a scalar, similar to species, so as well as convection it'll diffuse with the flow. A UDM is a calculated value in a cell, it can only "move" based on what maths creates it.
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