Questions regarding IDF docs description of esp_mqtt_client_init() config parameters

[MichaelS]

Hi
Can someone provide a more helpful description than the docs have of the MQTT esp_mqtt_client_config_t used in esp_mqtt_client_init(), specifically the following 3 parameters:
struct buffer_t

• size //size of MQTT send/receive buffer

• out_size // size of MQTT output buffer

struct outbox_config_t

• limit //Size limit for the outbox in bytes

The descriptions are vague indicate overlap in functionality.
Is the outBox only used for holding esp_mqtt_client_enqueue() messages and esp_mqtt_client_publish() messages bypass the outBox
What is the difference between size (send/receive buffer) and out_size (output buffer) and how are they used?
The terminology is confusing, is output used for send and size used for receive?
Thanks