What actually determines a step child or foster child for a dependant

[rgtaxes]

What is the legal document need for a step child or foster child 

[Opus 17 · Accepted solution]

A step child is a legal child of your spouse.  The document you need is a marriage certificate.  If your spouse has a legal child (biological, adopted, or step child from a previous marriage), that child becomes your step child upon your marriage.

A foster child is a child placed in your home by an agency that is legally allowed to make foster child placements under the laws of your state.  In some states, only a state agency or family court can do that. In other states, private placement agencies might also be qualified.  You need to check the laws of your state.  You don't normally need legal documents to file a tax return but if you are audited or someone else claims the same dependent, then presumably there are court orders, letters or other documents authorizing the foster placement.