In Contact Center Management web page, under help Contact Center Performance Management Data Dictionary. Shows standard reports (chapter), what data and formula is used for each field on a report. Sometime it is helpful to follow on step further back and look in the data dictionary for the table and field(s) definition and calculation method. The relevant table is provided in the standard report definition.
For service level it will involve threshold settings, so you will need to know what setting you are using for the appropriate skillset or application.