The first category would include behaviors that may lead to tissue dammage or potentially could in future lead to injury to the child or others. Some examples of these behaviors include self-injury, aggression or Pica. In the case of self-injury the child may be hitting his head or other body parts and cause medical conditions. In the case of aggression the child may injury himself or cause tissue dammage to others. These types of behaviors should be addressed and should be prioritized when there are many behavior problems to address.
The second group of factors to consider is whether the increase of decreas of the behavior tageted will lead to a more normal life for your child. For example, toilet training is an essential skills to learn in many cases in order to be admited to a school or grade level. Also, the lack of this skill may lead to placements in special education classes or necessitate special training during the school hours. Also, talking may be considered an essential skill to learn i possible. A child that does not talk will usually be placed in a special education class or will not be able to communicate effectively enought to function in a normal classroom.
Finally, one should especially consider prioritzing behavior which are not considered scoaill acceble in society or in many social sitautaions. For example, one should consider a behavior such as screamming, stealing or hurting others as a priority since this may lead to social disaproval or even legal consequences to the child in the future.