I want to get a list of all boundary elements belonging to a certain ElementMarker of a boundary mesh.
This code creates a simple mesh and allows to inspect the ElementMarkers of the boundary mesh.
<< NDSolve`FEM` cubi = Cuboid[{0, 0, 0}, {1, 1, 1}]; mesh = DiscretizeRegion[cubi, MaxCellMeasure -> 1]; bmesh = ToBoundaryMesh[mesh]; (*inspect element markers*) groups = bmesh["BoundaryElementMarkerUnion"] temp = Most[Range[0, 1, 1/(Length[groups])]]; colors = ColorData["BrightBands"][#] & /@ temp; surfaces = AssociationThread[groups, bmesh["Wireframe"[ElementMarker == #, "MeshElementStyle" -> FaceForm[colors[[#]]]]] & /@ groups]; Manipulate[ Show[{bmesh["Edgeframe"], choices /. surfaces}], {{choices, groups}, groups, CheckboxBar}, ControlPlacement -> Top ] 
With bmesh["Wireframe"[ElementMarker == 1]] I can visualize a specific boundary part: 
While bmesh["BoundaryElements"] returns all the triangles of the boundary:
{TriangleElement[{{5, 6, 1}, {5, 7, 6}, {4, 3, 8}, {4, 1, 3}, {4, 8, 1}, {2, 3, 1}, {2, 7, 3}, {2, 6, 7}, {2, 1, 6}, {8, 3, 7}, {5, 1, 8}, {5, 8, 7}}, {1, 2, 3, 4, 5, 4, 6, 6, 1, 3, 5, 2}]} How can I use something like bmesh["BoundaryElements"] in a way that it gives me all the boundary elements that belong to a certain ElementMarker?
