How does the law define primary care giver in child custody cases?

the primary care giver is the parent the child or children lives with. the one granted sole physical custody.How does the law define primary care giver in child custody cases?
The primary care giver is the person who the child spends most of the time with. that cares for them. Like in my case I'm a step mom. the mother has supervised visitation and the father has custody but a truck driver, so the courts named me the Primary care giver since I took them to school cooked diner b-fast helped with homework and took care of them most of the time. hope this helps good luckHow does the law define primary care giver in child custody cases?
It is the person that the child lives with on a day to day basics