Mesh Defaults
Use a Mesh Defaults node  to customize the mesh generation if your physics controls the mesh. One Mesh Defaults node is available for each physics or multiphysics interface.
To add a Mesh Defaults node, first add a Physics Interface or Multiphysics Interface then,
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On the Physics Interface toolbar, click the Mesh Defaults button , or
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Right-click the Physics Interface or Multiphysics Interface node and select it from the context menu.
Right-click the Mesh Defaults node to add a Usage Condition subnode if you need to vary the mesh generation depending on space dimension or user input value. Also right-click to add a Mesh Size subnode or a Boundary Layers subnode.
Meshing in the COMSOL Multiphysics Reference Manual.