In the development environment you can expose user actions owned by your eSpace to be used by a different one. In this context, eSpaces are called Producer and Consumer, respectively.
In the Producer eSpace, set the Public property of your action to Yes and publish the eSpace. How?
You cannot expose an action of your eSpace when:
Your eSpace is multi-tenant.
In these situations, an error message is presented and the eSpace is not valid.
Extensions are also Producers. Their actions are automatically exposed when the connected user has access rights to the Extension itself.
Use Action References | Action Properties