There are different definitions of downtime. The Software Update Manager technical downtime is often used to compare different downtime optimization techniques, because there's no manual interaction. So, there's a higher reproducibility and it's independent of manual steps and human speed.
From the point of view of the users, the business downtime is the most important downtime definition.
Business downtime is the duration in which a user can't use the SAP systems applications in production.
Why downtime?
An advantage of SAP's installation technology is that it allows customers to adapt, extend, and modify SAP software. During the upgrade process, these extensions will be kept and adjusted to the new release. Most of the required processing steps can be performed during SAP system uptime.
Downtime is necessary whenever you replace live running transactions with new functionality, and a potential risk of data inconsistency is given, such as changing the processing logic, or changing the data model or structure.
Overall guidelines:
Avoid frequent downtime with long downtime period.
Minimize frequent events related to downtime.
Avoid offline backups.
Reorganize databases only when necessary.
