Good Morning Don,
A casino can retire a "series" or it can "retire" a denomination within a series. It simply means taking the individual denomination or denominations off the floor and placing them in the main cage and/or vault. The checks could be brought back at anytime for use.

When a series (individual or collectivly) becomes obsolete it will no longer be used anywhere within the casino. Whether the series has been retired, destroyed, etc. has no affect upon its obsolescence.

Hope this helps.
Jim