A more thorough explanation:

In property law, a remainder subject to open is a type of future interest that is left to a class of people, some of whom are alive at the time of conveyance, but which can be diminished by others joining the class later on.

For example, if a will leaves an estate to an heir and their heir's children, the children who are alive at the time of conveyance have an interest subject to open. This means that any other children born after the conveyance will also share in the interest.

It's important to note that a remainder interest is vested (subject to open) if at least one member of the class is alive at the time of conveyance. However, this type of interest should not be confused with a contingent interest, which is subject to defeasance if certain conditions are not met.

For instance, if a will states that a child must survive the life tenant in order to acquire interest in the property, then the child's interest is contingent and subject to defeasance if they do not meet the condition of survival.

Overall, a remainder subject to open is a way to ensure that future generations of a family or group can benefit from a property interest, even if they are not yet born at the time of conveyance.