A more thorough explanation:

Secta unica tantum facienda pro pluribus haereditatibus is a Latin legal term that means "one suit alone to be performed for several inheritances." It refers to a writ that exempts the eldest heir from performing all services but one, when they are distrained by a lord to perform several services for the coheirs.

For example, let's say that a lord has several coheirs who are entitled to inherit his land. The lord may require the eldest heir to perform several services, such as paying rent or providing labor, in order to receive their inheritance. However, if the eldest heir is unable to perform all of these services, they may be able to use the writ of secta unica tantum facienda pro pluribus haereditatibus to only perform one service and still receive their inheritance.

Another example could be a situation where a family member passes away and leaves multiple properties to their heirs. The eldest heir may be required to perform certain tasks, such as paying taxes or maintaining the properties, in order to receive their inheritance. If the eldest heir is unable to perform all of these tasks, they may be able to use the writ of secta unica tantum facienda pro pluribus haereditatibus to only perform one task and still receive their inheritance.

These examples illustrate how the writ of secta unica tantum facienda pro pluribus haereditatibus can be used to exempt the eldest heir from performing all services but one, when they are distrained by a lord to perform several services for the coheirs or when they are required to perform certain tasks in order to receive their inheritance.