Some feedback:
I got confused by the wording of the condition "Old event tag Service = new event tag Service", since it was not clear to me that it compares tag name AND its value.

Maybe the docs should be more clear when a "tag name", "tag value" or "tag name" AND "tag value" are compared.

From the docs:

Old event tag - match new event to the old event(s) that has the corresponding old event tag
New event tag - match new event that has the corresponding event tag to old event(s)
New event host group - match new event that belongs to the corresponding host group to old event(s)
Event tag pair - match new event to the old event(s) if the values of the specified tag in both events match
Old event tag value - match new event to the old event(s) that:
= - has the corresponding old event tag value

https://www.zabbix.com/documentation/3.4/manual/config/event_correlation/global

Maybe we could improve it, maybe even not only in documentation, but for strings in frontend too.