A more thorough explanation:

An ulterior remainderman is a person who holds or is entitled to receive a remainder in a property, but only if all the preceding reciprocal interests expire. This means that their future interest in the property will only take effect if certain conditions are met.

For example, let's say that John owns a piece of land and gives it to his son, Tom, for his lifetime. After Tom dies, the land will go to his daughter, Sarah. However, if Sarah dies before Tom, then the land will go to John's nephew, Mark. In this scenario, Mark is the ulterior remainderman because his future interest in the land is contingent upon Sarah's death before Tom.

Another example could be a trust that is set up to provide income to a beneficiary for their lifetime. After the beneficiary's death, the trust assets will go to their children. However, if the beneficiary has no children, then the assets will go to a charity. In this case, the charity is the ulterior remainderman because their future interest in the trust assets is dependent on the beneficiary not having any children.