There are various ways to share custody in a divorce, and they differ slightly between physical custody (parenting schedule) and legal custody (parental decision-making).
Joint legal custody means that the parents not only have to share information about the child and discuss all major decisions concerning the child, but they also have to come to mutual agreement on a decision before taking it.
For legal custody, there’s an important distinction between having joint or sole [legal] custody. If a parent has sole legal custody, they have the final say (in a disagreement) on major decisions concerning the child. Sole legal custody does not mean that the custodial parent is entitled to exclude the other parent from the decision-making process, or to not share information about the child with the other parent. It simply means that in the event of a disagreement (e.g., Mom wants son to go to X school, and Dad thinks Y school would be best), the parent with sole legal custody will have the final say.
As you might imagine, most parents don’t like the idea of the other parent having sole legal custody. As a result, joint legal custody is far more common, in particular among the roughly 95% of divorcing couples who settle their divorce. Joint legal custody basically means that the parents not only have to share information about the child and discuss all major decisions concerning the child, which they do with sole legal custody, as well—but they also have to come to mutual agreement on a decision before taking it.
Many parents going through divorce are concerned about their ability to come to mutual agreements regarding their kids—perhaps on a specific topic that they see differently, or perhaps because of a general breakdown in their communication. In those instances, parents may choose to specify (in their settlement agreement) a method for resolving disputes between them regarding their kids. Examples of this might be committing to at least one session with a mediator or neutral professional to attempt to resolve their dispute, or giving each parent the final say over certain types of decisions (e.g., Dad has the final say on health-related decisions, and Mom has the final say on educational decisions).
For physical custody, which is basically a parenting schedule for your kids, the language in your ultimate agreement will differ depending on the type of parenting schedule that works for your family.
Where you’ve chosen a 50-50 schedule with your kids, you will most often say that the parents have “joint physical custody,” and you may say that they are equally the primary residential parent, that neither is the primary residential parent, or you may not speak at all to a “primary residential parent.”
Where you’ve chosen a schedule that has the kids living more than half of the time with one parent, you’ll typically name that parent as the “primary residential parent.” Beyond that, legal practice differs somewhat in terms of how your arrangement is described in your legal agreement. Some people will say that the parents have “joint physical custody” and that Mom/Dad is the “primary residential parent.” Others won’t speak explicitly to “physical custody,” but will say that Mom/Dad is the “primary residential parent,” and that the parents will share parenting time, or that Dad/Mom will have parenting time (or “visitation”), as set forth in the agreement. It’s less common to say that the parent who has the children more than half of the time has “sole physical custody” of them.
You don’t need to commit any of the above terms to memory. It’s much more important to figure out, practically speaking, which kind of parenting schedule and decision-making structure will work for you and your ex going forward. Once you’ve done that, you can work with your attorney or mediator to understand how to best describe what you’ve come up with in your settlement agreement.