Clarify custom hostname routing for Worker as origin (#27424)

* Clarify custom hostname routing for Worker as origin

Addresses the lack of clarity about how custom hostname traffic reaches Workers when using Cloudflare for SaaS.

Changes:
- Add new section explaining how custom hostname traffic flows through the SaaS zone
- Provide concrete example showing the relationship between SaaS zone, fallback origin, custom hostname, and Worker route
- Clarify that wildcard route pattern (*/*) automatically captures custom hostname traffic
- Rewrite step 4 with clearer routing options and examples
- Add related resources section linking to hostname routing and Workers routes docs

Fixes #27397

* Added case of only some hostnames to worker (#27425)

* Added case of only some hostnames to worker

* Removed copied line

---------

Co-authored-by: Isak Wang Gustavsen <69854945+isakgustavsen@users.noreply.github.com>

src/content/docs/cloudflare-for-platforms/cloudflare-for-saas/start/advanced-settings/worker-as-origin.mdx

@@ -7,32 +7,55 @@ head:
   - tag: title
     content: Workers as your fallback origin
 description: Learn how to use a Worker as the fallback origin for your SaaS zone.
-
 ---
 
 import { Render } from "~/components";
 
 If you are building your application on [Cloudflare Workers](/workers/), you can use a Worker as the origin for your SaaS zone (also known as your fallback origin).
 
-1. In your SaaS zone, [create and set a fallback origin](/cloudflare-for-platforms/cloudflare-for-saas/start/getting-started/#1-create-fallback-origin). Ensure the fallback origin only has an [originless DNS record](/dns/troubleshooting/faq/#what-ip-should-i-use-for-parked-domain--redirect-only--originless-setup):
+## How custom hostname traffic reaches your Worker
 
-   * **Example**: `service.example.com AAAA 100::`
+When customers point their domains to your SaaS zone (for example, `mystore.customer.com` CNAMEs to `service.saasprovider.com`), their traffic enters your Cloudflare zone. Any Worker routes configured on your zone will match this incoming traffic.
+
+For example, if you have:
+
+- Your SaaS zone: `saasprovider.com`
+- Your fallback origin: `service.saasprovider.com`
+- Customer's custom hostname: `mystore.customer.com` (pointed to your zone via CNAME)
+- Worker route: `*/*`
+
+When a visitor requests `mystore.customer.com`, Cloudflare routes that request through your zone. The `*/*` route pattern matches all traffic entering your zone, including traffic from custom hostnames like `mystore.customer.com`.
+
+:::note
+You do not need to add individual Worker routes for each custom hostname. The wildcard route pattern (`*/*`) automatically captures all traffic entering your zone, including traffic from customer vanity domains.
+:::
+
+## Set up a Worker as your fallback origin
+
+1. In your SaaS zone, [create and set a fallback origin](/cloudflare-for-platforms/cloudflare-for-saas/start/getting-started/#1-create-fallback-origin). Ensure the fallback origin only has an [originless DNS record](/dns/troubleshooting/faq/#what-ip-should-i-use-for-parked-domain--redirect-only--originless-setup):
+   - **Example**: `service.example.com AAAA 100::`
 
 2. In that same zone, navigate to **Workers Routes**.
 
 3. Click **Add route**.
 
-4. Decide whether you want traffic bound for your SaaS zone (`example.com`) to go to that Worker:
+4. Configure a route to send traffic to your Worker. Choose one of the following options based on your needs:
+   - **Route all traffic to the Worker** (recommended for most SaaS applications):
+     - **Route**: `*/*`
+     - **Worker**: Select the Worker used for your SaaS application.
 
-   * If *yes*, set the following values:
+     This pattern routes all traffic entering your zone to the Worker, including requests from custom hostnames (for example, `mystore.customer.com`) and requests to your own subdomains (for example, `app.saasprovider.com`).
 
-     * **Route**: `*/*` (routes everything — including custom hostnames — to the Worker).
-     * **Worker**: Select the Worker used for your SaaS application.
+   - **Route all but specific routes to worker**:
+     - **Route**: `*/*`
+     - **Worker**: Select the Worker used for your SaaS application.
+     - Add a second route for your zone's own hostnames with **Worker** set to **None** to exclude them.
 
-   * If *no*, set the following values:
+     For example, if your zone is `saasprovider.com` and you want `api.saasprovider.com` to bypass the Worker, create an additional route `api.saasprovider.com/*` with no Worker assigned. More specific routes take precedence over wildcard routes.
 
-     * **Route**: `*.<zonename>.com/*` (only routes custom hostname traffic to the Worker)
-     * **Worker**: **None**
+    - **Route only custom hostname traffic to the Worker**:
+     - **Route**: `vanity.customer.com`
+     - **Worker**: Select the Worker used for your SaaS application.
 
 5. Click **Save**.
 
@@ -43,3 +66,8 @@ If you are building your application on [Cloudflare Workers](/workers/), you can
 <Render file="same-name-as-zone" product="cloudflare-for-platforms" />
 
 :::
+
+## Related resources
+
+- [Hostname routing](/cloudflare-for-platforms/workers-for-platforms/configuration/hostname-routing/) - Learn about advanced routing patterns, including dispatch Workers and Orange-to-Orange (O2O) behavior.
+- [Workers routes](/workers/configuration/routing/routes/) - Learn more about route pattern matching and validity rules.