PGD is recommended to identify embryos that are at risk for a single gene disorder, e.g. cystic fibroses, and for testing for chromosomal structural rearrangements, e.g. translocations, deletions/duplications. PGS or now called PGT, preimplantation genetic testing, is used to detect aneuploidy.
Suggested for you: What Is PGD or Preimplantation Genetic Diagnosis?.