What is X++?
X++ is the object-oriented programming language used to customize Dynamics 365 Finance and Operations apps, as it was in Dynamics AX. Developers deliver X++ changes as extensions, because changing Microsoft's standard code, known as overlayering, is not supported in the cloud versions.
Why it matters for your business
Extensions keep the system updatable under Microsoft's continuous update model. Many needs can also be met with configuration, Power Platform apps or ISV solutions, which can be easier to maintain than custom X++.
A manufacturer adds a custom field and validation rule to production orders through an X++ extension, so the next service update does not overwrite the change.