https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClomCAC

By default, the firewall implicitly allows intrazone traffic (within a zone) and implicitly denies interzone traffic (between zones).
By default, traffic allowed or denied by the implicit Security policy rules is not logged on the firewall.

Universal -By default, all the traffic destined between two zones, regardless of whether it is from the same zone or different zone. Universal rule types apply to all matching interzone and intrazone traffic in the specified source and destination zones.