An open work item is a work item that is not yet resolved, or was reopened after it was previously resolved. The property Work-.pyStatusWork determines whether a work item is open or resolved. Any status value that does not start with the literal text Resolved is an open status value.
Open selected item means to open selected work item.
Open Selected Item action belongs to "List" category, which means this particular action should be used in the purview of repeating layouts. And what it exactly means is that if we have a Repeating Grid displaying a list of Assignments or Work Objects, we would be able to open the same in a new tab process them accordingly.
Now on the contrary Open Work Item action belongs to "Get Work" category and it is used to open any unresolved work object based on the work id parameter.