What is AL extension?
An AL extension is a package of custom code, written in Microsoft's AL language, that adds features to Dynamics 365 Business Central without changing its standard code. Extensions survive Business Central updates.
Why it matters for your business
Building customizations as extensions keeps Business Central upgradeable. Older systems modified the core code directly, which made every upgrade slow and risky.
A wholesaler adds a custom approval step and a branded delivery note to Business Central through one AL extension, and both keep working after Microsoft's monthly updates.