Iirc, some gaming commissions require casinos to have a secondary set of chips in storage that can be quickly swapped out if they believe the primary chips to be compromised.
Denominations will be the same, but typically quantity is slightly less since they aren’t intended to be in play for long periods of time.
As it pertains to collectors, secondary chips tend to be in new or very good condition because they don’t usually ever see the casino floor. This, combined with scarcity compared to the primaries, typically makes them a little pricier.
Hope this helps!