-*tag*: a positive integer associated to this domain. If only one domain is available, this number can be choose freely as a positive integer. When multiple domains are presents, this number provides different possibility:
* If *fdg=False*, and *tag* is the same for all domains, a conventional constitous Galerkin (CG) FE is found.
* If *fdg=False*, and *tag* is the same for all domains, a conventional constitous Galerkin FE (CG) is found.
* If *fdg=False*, and *tag* is different between two domains, CG is used for these domains but an DG interface between these two domains must be provided to guarantee the continuity of the fields accross this interface. This option is useful when considering multiple domains when only decohesion between them is interested.
-*phys*
-*sp*
* If *fdg=True*, *tag* can be assigned to an arbitrary positive integer as a full discontinuous Galerkin (DG) is considered.
-*phys*: Finite elemet support of this domain. The value of *phys* corresponds to the *Physical* number of the domain defined in the geometry with Gmsh.
-*sp*: function space type governs the finite element approximation: