The "subscriptionActions" property adds or removes a topic filter from a client's subscriptions. Each subscription object contains a "topicFilter" and an optional "subscriptionIdentifier".

The "includeExpiredMessages" property includes expired messages in a subscription when set to true. A message has expired when the "messageHasExpired" property in the response is true.

The "receiveMyMessages" property allows a client's subscription to receive messages published by the client.

The "receiveRetainedFlagAsPublished" specifies how the FairCom server sets the "retain" property on messages it sends to subscribers. When a client publishes a message, it sets the "retain" property to true to tell the FairCom server that it should retain the message and publish this message on behalf of the client when another client subscribes to a topic. A retained message is typically a special message that provides a new subscriber with status information about the topic.

When "receiveRetainedFlagAsPublished" is true, the FairCom server follows the MQTT 3 specification and passes the value of the "retain" property set by the publisher to the subscriber. Thus, a subscriber sees the value of the "retain" property that is set by the publisher. This notifies the subscriber that the message is a special status message from the client. This approach has the downside that a subscriber does not know when it gets a message sent by the FairCom server on behalf of the client in response to subscribing to a topic.

When "receiveRetainedFlagAsPublished" is false, the FairCom server follows the MQTT 5 specification and sets the "retain" property to true only when the FairCom server sends a retain message to a subscriber on behalf of a publisher, such as when a client subscribes to a topic. The MQTT 5 approach lets a subscriber know when it receives a retain message sent by the FairCom server vs. when it receives a normally published message from a client. This approach has the downside that a subscriber does not get the value of the "retain" property as set by the client who published the message. The MQTT 5 approach is generally better because the subscriber can tell when it is receiving a retained message in response to a subscription as opposed to a message published by a client.

The "receiveRetainedMessages" property specifies how MQTT 5 subscribers receive retained messages. It has three enumerated values: "onSubscription", "onNewSubscription", and "never". 

The "subscriptionIdentifier" property is provided by the subscriber and uniquely identifies the "topicFilter" to the subscriber. Each subscriber may assign its own "subscriptionIdentifier" to each "topicFilter". When the FairCom server delivers a message to the subscriber, it includes the "subscriptionIdentifier" with the message. The purpose of Subscription Identifiers is to make it easy and fast for an MQTT 5 client to process an incoming message. The client can use the Subscription Identifier in a message to associate a message with a specific handler. A client receiving a message with multiple Subscription Identifiers can send the message to multiple handlers – one for each Subscription Identifier.

The "topicFilter" property contains a topic filter, which may contain MQTT wildcard characters, #, and +. Without wildcard characters, it matches one topic. When containing wildcard characters, it may match one or more topics.

The "userProperties" property specifies an array of key-value objects used to describe a user. A key-value object contains a "key" property and a "value" property with string values. You can assign multiple key-value pairs to an MQTT 5 message; the server includes them in the messages it delivers to subscribers. The server may process specific key-value pairs as instructions that modify its behavior.

"userProperties": [
  {
    "key": "some key",
    "value": "some value"
  }
]

The "topicFilter" property contains a topic filter, which may contain MQTT wildcard characters, #, and +. Without wildcard characters, it matches one topic. When containing wildcard characters, it may match one or more topics.