Extensions

Extensions are the modern, supported way to customise Microsoft business apps. In Business Central, an extension is an AL package that adds new tables, pages, code, and integrations alongside the base application without modifying Microsoft's own objects — which is why upgrades stay non-breaking. Extensions are installed per tenant, published through AppSource, or pushed by a partner. Dynamics 365 CRM-side apps use a similar concept through solutions in Dataverse, which package tables, forms, security roles, flows, and Power Apps. Both models replace the older practice of editing the base product directly.

In practice, "extend, don't modify" is enforced by the platform itself, not just convention: a Business Central extension can only touch the base application through defined extension points — event subscribers, table/page extension objects — and simply has no mechanism to edit Microsoft's own base objects directly, unlike the old NAV practice of copying and modifying standard code (C/AL customisation). This is precisely what makes a well-built extension survive a platform upgrade unattended, where an old-style direct modification would have to be manually re-applied and re-tested against every new release.

Where confusion shows up: a per-tenant extension (PTE) and an AppSource app are both "extensions" technically but follow very different governance — a PTE is installed directly into one tenant by a partner with no Microsoft review, while an AppSource app must pass Microsoft's validation and is available to any customer. The choice between them is a distribution decision (one customer's bespoke need versus a packaged product for many), not a technical one — the same AL code can usually be shipped either way.