Occurrences error budget calculation method
The Occurrences method of error budget calculation counts good attempts (for example, requests that are within defined boundaries) against the count of all attempts (i.e., all requests, including requests that perform outside of the defined boundaries).
This method is well suited to measuring recent user experience. Since there are fewer total attempts during low-traffic periods, it automatically adjusts to lower traffic volumes.

Target
Target is used to calculate your error budget. If you want something to be reliable 95% of the time (or in other words, have at most a 5% failure rate over the defined time window), then your target should be set at 95%. This is the reliability target for the service level objective (SLO).
For the Occurrences method, the target defines the percentage of good events to total events that meet the defined target.
