A more thorough explanation:

Domestic partner adoption is when one partner in a domestic partnership adopts a child born to the other partner. Domestic partnerships are relationships between two people who do not want to get married or cannot get married, such as same-sex couples. In some states, domestic partners have similar rights to married couples.

In states where domestic partnerships are recognized, one partner can adopt the biological child of the other partner. This means that the child will have two legal parents, and the adoptive parent will have the same rights and responsibilities as the biological parent. However, in most states, a child can only have two legal guardians, so if there are two biological parents, a third domestic partner cannot adopt the child.

The biological parent in the domestic partnership must agree to the adoption, but they can still retain their full parental rights to the child. After the adoption, the child will have two legal parents, and the adoptive parent will have the same joint-custody rights and financial obligations as the biological parent.

For example, if a same-sex couple in California has a child, one partner can adopt the child so that they both have legal parental rights. If the couple separates, the adoptive parent will still have joint-custody rights and financial obligations to the child.

In states where domestic partner adoption is not allowed, couples may opt for a co-parenting or custody agreement. This allows them to share rights and responsibilities regarding the child, but only the biological parent will be considered the legal parent.

For example, in New York, a court case called B.S. v. F.B. allowed a lesbian couple to share custody of their child through a co-parenting agreement, even though only one of them was the legal parent.