They are those who have explicitly requested not to receive more communications from you.

You can use our Unsubscribe URL in each of your communications for this purpose: If the end users click on it, they will be unsubscribed.

Within your audience, the Subscribed to SMS attribute will have a value of 'no'. (Remember that you must select this attribute in the ⚙ icon to make it visible on the screen)

When preparing a campaign, you will need to decide whether to keep this option checked:

Keeping this option always checked is highly recommended as it will exclude those contacts from the audience you have selected.

You may also be interested in knowing the difference between the SMS acceptance attribute and the SMS subscription attribute.