Structured meshing and Sweep meshing technique results in a ordered sequencing of the elements. Say, if you have a rectangular part (with no cutouts or without any stress risers), you can use a structured mesh in this case. Say, if you have a small circular cutout (or a crack), you can have rings of nice looking elements around the crack tip or the circular cutout (by using a swept meshing technique).
Adaptive remeshing needs to refine the mesh, in the region where the error variable reports a higher value and needs to coarsen the mesh where the error variable reports a smaller value. So, it needs freedom to control the mesh. The 'Free' meshing technique gives this freedom (since it results in a random mesh and there is no ordered logic in a free mesh), and this is something that adapative remeshing can work with easily and can move around the elements.
When it creates new elements in a region of stress concentration, you cannot always ask it to fill it up with only hex or quad elements (if 2D). It is more easier to fill up the gap using tri or tet elements. Remember that tri and tet elements can fill up any complex shape, but hex or quad cannot.
Best Regards,
Vishakh Rajendran.
