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Important: Mesh generation does not depend on the state of the layers when the generation is performed (see Layers and groups (only Preprocessing)) 2438267245). All layers are meshed and every node and element will be assigned to the layer where the original geometrical entity was. The only exception to this rule is in the case of a frozen layer if the No mesh frozen layers option is selected (see Preferences).
Groups:
Groups is a concept similar to layer, but the big difference is that each entity must belong to one and only one layer, but can belog belong to an arbitrary number of groups
Layers are useful to create and modify the geometry. Groups are used as a collection of entities to apply simulation conditions, like forces, velocities or other properties. 
The commands related to groups are
- New group: Creates a new group with a name
- New child group: Creates a new group and set as a child of the currently selected group.
- Delete: Deletes a group and the list of entities that belong to it, but the entities are not deleted.
- Rename: change the name of the group
- Assign: add geometry or mesh entities of the specified type to the collection of the group
- Unassign: remove enties entities of the group
- Draw groups by color: Temporary draw by color the entities of the selected groups. The color and its transparency is a property of the group.
- List group entities: show the list of entities by type that belongs to the selected group