Sometimes the court looks at all of the evidence and still does not feel that they can truly determine the best interest of the child. Due to the need for more information, they order a 730 Evaluation. A marriage and family therapist who has does some other special training and approved by your county courts will meet with both parents, the child, and anyone else that they deem necessary (teachers, half siblings, doctors, etc). They will also review report cards, letters from friends, medical records, and any other pertinent documents.
Although a wonderful way to ensure that the child is met with and all of the information is on the table, these can be incredibly expensive. If you can get a focused evaluation - meaning they look at just the move away and not custody as a whole (or whatever other issue you have in front of the court) you can usually find a flat fee of about 3,000-5,000. If you do an ENTIRE evaluation they can cost 10k-20k.
Due to the cost people try to avoid them wherever possible. However the court and mediators if you are in a recommending county, can request one if they feel that it is needed.