Explanation/Reference:
E: Event Levels
Warning, Level ID 50
Information, Level ID: 80
Verbose, Level ID: 100
F: Trace levels:
Monitorable: 15
Medium: 50
Verbose: 100
Note: When using the Unified Logging System (ULS) APIs to define events or trace logs, one of the values you must supply is the ULS level. Levels are settings that indicate the severity of an event or trace and are also used for throttling, to prevent repetitive information from flooding the log files.
Reference:https://msdn.microsoft.com/en-us/library/office/ff604025(v=office.14).aspx