Microsoft
Microsoftpschum in the context of Monthly Enterprise Channel, there will only ever be 1 Feature Update and 1 Extended Quality Update at any given time, and the titles will be unique between the two. That said, if you need to remain on the extended release of MEC then you would not want to push any updates with Feature Update in the title. This would keep you at N-1 from an update perspective.
The Extended Quality Updates are not a 1:1 of the previous Feature Update. They are a rev'd version of the previous Feature Update, including current quality and security updates. For example, the EQU for July and August 2021 included additional security updates, while the EQU for June 2021 included additional quality fixes and security updates.