You can set up additional methods to track more than the response and resolution times on tickets. For instance, maybe you would like to track the amount of time that an SLA was in the Waiting on Vendor status. In this example, you would begin by clicking on Custom Metric in the top right corner of the SLA Details page of any SLA.

This will open the custom metric window. You will need to name your new metric as well as provide a target time. Once you have filled in these fields, click on the Add Filter option on the right of the Timer is running when section.
For this type of set up, you will want to select the Status filter type. From here you can select the status, or statuses, that you would like to include or exclude. For the example below the Waiting on Vendor status is selected.
You will now see your new custom metric added underneath the response and resolution times. Be sure to click Save before leaving this page to ensure your new SLA settings persist.