A more thorough explanation:

A testamentary class is a group of beneficiaries who are uncertain in number but will be ascertainable in the future, when each will take an equal or other proportionate share of the gift. This term is often used in the context of wills and trusts.

For example, a person may leave a gift to "all of my grandchildren who are alive at the time of my death." At the time the will is written, it is uncertain how many grandchildren will be alive when the person dies. However, once the person passes away, the number of grandchildren who are eligible to receive the gift becomes ascertainable.

Another example of a testamentary class is a gift to "all of my employees who have worked for me for at least five years." At the time the will is written, it is uncertain how many employees will meet this requirement. However, once the person passes away, the number of eligible employees becomes ascertainable.

Overall, a testamentary class is a way to ensure that a gift is distributed fairly among a group of people who share a common characteristic or attribute, even if the exact number of beneficiaries is unknown at the time the will or trust is created.