I think the answer hides behind the meaning of the words. As you know "In database systems, isolation is a property that defines how/when the changes made by one operation become visible to other concurrent operations.-WikiPedia".
So far I know, when IsolationLevel is ReadCommited on Transaction can be viewed in another Transaction. When it's None, it can not be seen.
When IsolationLevel is Serializable, the isolation level specifies that all transactions occur in a completely isolated fashion; i.e., as if all transactions in the system had executed serially, one after the other(wiki).