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As far as I understand it, it's an internal currency only. But you can give away the ESTM (to other users), you can boost a post (with a curation) with it and you can use it to promote a post.
There seem to be some rules for promoting and boosting posts. In example, the respective post must have been made with the Esteem app and the post must be new.

You can buy upvotes or you can promote your post on Esteem (now ecency) for your ESTM tokens.