two of them with the same contents are interchangeable, so you never update one — you replace it.

Value object

Also calledvalue type, immutable value

An object defined entirely by its attributes, compared by value and, by convention, immutable: money, a date range, a coordinate, an email address. Immutability plus value equality means it can be shared freely, used as a map key and reasoned about without aliasing. Most of the practical benefit of domain modelling comes from making more things value objects, because each one takes a validation rule out of the calling code and puts it somewhere it cannot be skipped.

Commonly confused with

Object equality
Embedded value
Entity
Tiny type
Data transfer object

Data transfer object vs Value object →
Embedded value vs Value object →
Entity vs Value object →
Object equality vs Value object →
Tiny type vs Value object →