add docs on line model

Author: qwerzl

docs-src/spectrum-ts/providers/imessage.mdx.vel

@@ -56,6 +56,25 @@ The iMessage provider supports three connection modes — local, cloud, and dedi
   </Tab>
 </Tabs>
 
+## Line model
+
+Cloud mode routes your messages through phone numbers ("lines") provisioned by Spectrum. Which lines you get depends on your plan, and the difference is mostly invisible to end users.
+
+| Plan | Line allocation | What end users see |
+|---|---|---|
+| **Free / Pro** | **Shared pool.** Each of your end users is routed through a different number from a shared pool. | A normal iMessage from a number that may differ across recipients. |
+| **Business** | **Dedicated.** All of your end users text the same number, which belongs to your project. | A normal iMessage, always from the same number. |
+
+End-user delivery is identical in both modes; the distinction is which number sends.
+
+### Auto-scale
+
+When traffic to a dedicated line approaches its per-line capacity, Spectrum can automatically provision an additional line so deliverability isn't affected. It's an opt-in feature on the Business plan. Enable it in your project settings if you'd rather not get paged when a line saturates.
+
+<Note>
+  These are managed Spectrum Cloud features. If you're on the open-source path (`imessage.config({ local: true })` or your own dedicated relay), you provide your own iCloud account and managed-line concepts don't apply.
+</Note>
+
 ## Space types
 
 iMessage spaces carry a `type` field — `"dm"` or `"group"` — accessible through narrowing: