While deciding between PubSub and PubSub Lite, I came across the point where it says "... can work with availability risks inherent to zonal services? Use zonal Lite topics". Source at Choosing Pub/Sub or Pub/Sub Lite.
What exactly are these availability risks?
A regional service like Cloud Pub/Sub is resilient to individual zones being out because there are multiple zones in a region. Therefore, if one of those zones is unavailable for any reason, whether it be in Pub/Sub itself or any of its underlying internal dependencies, it should still behave as expected without any steps required on the part of the user.
On the other hand, a zonal service doesn't have that resiliency against a zonal outage: if the zone where your topic lives is unavailable, then that topic and its messages are unavailable until the zone recovers.