What is the best definition of the Heartbeat Interval?

The firewalls exchange hello messages and heartbeats at configurable intervals to verify that the peer firewall is responsive and operational. Hello messages are sent from one peer to the other to verify the state of the firewall. The heartbeat is an ICMP ping to the HA peer. A response from the peer indicates that the firewalls are connected and responsive.

https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClUcCAK

"A "heartbeat-interval" CLI command was added to the election settings for HA, this interval has a 1000ms minimum for all Palo Alto Networks platforms and is an ICMP ping to the other device through the HA control link." https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000ClMaCAK