The answer is "not well defined, clearly understood or fully agreed upon".
One of the results of the high rate of separation and remarriage is that family structure has changed. Individuals who remarry end up mixing two arrangements of families from previous relational unions. That implies that the recently remarried are presently both proceeding to be the regular parent to their current children and step parent to the kids who accompany the second life partner. Once in a while it is just a single life partner who carries kids into the marriage.