typically

This word itself indicates that there is a usual use and non usual use. Usually it’s used for adoption but that’s not the only circumstance

Here’s another page: A parent can sign an “affidavit of voluntary relinquishment” of parental rights if the parent agrees that a court should terminate his or her parental rights to a child.

Here’s an AVVO with an answer from a Nevada lawyer about the process

Heres from a lawyers website in CA

Second, a biological parent may choose to give up their rights if the parents don’t have a meaningful or ongoing relationship.

Third, the pregnancy may be unexpected and one parent opts to not be involved in the child’s life.