| A *union payload* is a FIDL method payload that uses the `union` layout. The |
| top-level type used as the method payload must use one of `struct`, `table`, or |
| `union` as its layout. Notably, some generated bindings "flatten" the arguments |
| passed to `struct` method payloads, such that each member is itself treated as a |
| function argument in the calling signature. Payloads that use `table` or `union` |
| never do this, and always pass a single argument, called `payload`, instead. |
| |
| <<../examples/key_value_store/_callout.md>> |
| <<../examples/key_value_store/_use_generic_values_tutorial.md>> |
