man mosquitto :

In addition to allowing clients to subscribe to specific topics, mosquitto also allows the use of two wildcards (+ and #) in subscriptions. + is the wildcard used to match a single level of hierarchy. For example, for a topic of "a/b/c/d", the following example subscriptions will match:

• a/b/c/d

• +/b/c/d

• a/+/c/d

• a/+/+/d

• +/+/+/+

The following subscriptions will not match:

• a/b/c

• b/+/c/d

• +/+/+

The second wildcard is # and is used to match all subsequent levels of hierarchy. With a topic of "a/b/c/d", the following example subscriptions will match:

• a/b/c/d

• #

• a/#

• a/b/#

• a/b/c/#

• +/b/c/#

The $SYS hierarchy does not match a subscription of "#". If you want to observe the entire $SYS hierarchy, subscribe to $SYS/#.

Note that the wildcards must be only ever used on their own, so a subscription of "a/b+/c" is not valid use of a wildcard.

The # wildcard must only ever be used as the final character of a subscription.