The Flow Callout policy is used to invoke a Shared Flow from within an API proxy or another Shared Flow. It can be attached to the PreFlow, PostFlow, or any conditional Flow within a ProxyEndpoint or TargetEndpoint. It can also be used inside one Shared Flow to call another, a practice known as "chaining." The only location where it cannot be used is the PostClientFlow, which executes after the response is sent to the client and has a restricted set of allowable policies that does not include Flow Callout.

C. Only in the Flow phase of ProxyEndpoint or TargetEndpoint: This is incorrect because a Flow Callout can also be used in the PreFlow and PostFlow phases, not just conditional Flows.

D. Anywhere except another Shared Flow: This is incorrect. The official documentation explicitly states that Shared Flows can be "chained" by using a Flow Callout policy within one Shared Flow to execute another.

E. Only in ProxyEndpoint PreFlow or TargetEndpoint PostFlow: This is too restrictive. A common use case is to place a Flow Callout within a conditional Flow to execute reusable logic for a specific API resource.

1. Google Cloud

Apigee Documentation

"Shared flows": "You can also chain shared flows together

that is

have a Flow Callout policy execute another shared flow." This supports answer B and refutes answer D.

2. Google Cloud

Apigee Documentation

"Flow Callout policy": "You can use a Flow Callout policy in a ProxyEndpoint or TargetEndpoint." This general statement supports its use in various flow locations.

3. Google Cloud

Apigee Documentation

"Configuring flows"

Section: PostClientFlow: This section lists the specific policies that can be attached to the PostClientFlow. The FlowCallout policy is not on this list

which includes AccessControl

KeyValueMapOperations

MessageLogging

ServiceCallout

and StatisticsCollector. This supports the exclusion mentioned in answer A.