Volume meshes can be made out of different element types.
A lot of times pure hexahedrons or tetrahedrons are used.
The BLM-mesh is ideal for the exact display of the boundary area.
BLM stands for Boundary Layer Mesh.
The boundary layers are made out of prisms with triangular surface.
The inside is filled with tetrahedrons.
Especially in injection molding simulation, this mesh type is preferred.