One or more characters, space separated. A single asterisk (*) is a wildcard, which means all defined actions.
For a Multirow list with an action column, and no FixedAction defined, this provides a list of the default actions available in a dropdown.
Note that without user-written javascript extensions in the project's [projectname]rules.js file, you should not change this from the asterisk default.
When you do implement a function in the [projectname]rules.js file related to the subject list, you are able to alter the actions available based on values contained in the list (like not allowing a "D" for delete action if a particular member of the list row had a value over a particular limit). Altering the rules on a list is an advanced topic requiring strong javascript skills.
