The XL icon you see next to some of the markets in PackRat denotes a premium market that requires a paid subscription in order to access. While credits are still used to purchase cards in these markets, and while free users are welcome to browse these markets, free users cannot make purchases here without a subscription.
For more information on the different player account types, please see the following article.
The following four markets are XL only:
When a collection is just released, some cards are released first to XL (and ticket) markets before then becoming available in free credit markets after a certain amount of time (usually a few weeks or so).
After migrating to the credit market, cards will then retire permanently to a ticket-only market.