VK_QUEUE_FAMILY_EXTERNAL

The special queue family index VK_QUEUE_FAMILY_EXTERNAL represents any queue external to the resource’s current Vulkan instance, as long as the queue uses the same underlying device group or physical device, and the same driver version as the resource’s VkDevice, as indicated by VkPhysicalDeviceIDProperties::deviceUUID and VkPhysicalDeviceIDProperties::driverUUID.

#define VK_QUEUE_FAMILY_EXTERNAL (~1U)

or the equivalent

#define VK_QUEUE_FAMILY_EXTERNAL_KHR VK_QUEUE_FAMILY_EXTERNAL