Comparison to chain extension

examples/call-runtime/README.md

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   }
   ```
+
+## Comparison to `ChainExtension`
+
+Just as a chain extension `call_runtime` API allows contracts for direct calling to the runtime.
+You can trigger any extrinsic that is not forbidden by `pallet_contracts::Config::CallFilter`.
+Consider writing a chain extension if you need to perform one of the following tasks:
+- Return data.
+- Provide functionality **exclusively** to contracts.
+- Provide custom weights.
+- Avoid the need to keep the `Call` data structure stable.