How is Cover Calculated [SC] ?

How does StaffCover find the best person to cover for an absence or event ?

The short answer is that it applies the rules that you have set-up.  There are lots of possible rules for you to apply or customize ... or you can just keep StaffCover's defaults.
For more information, see:

-- Cover Parameters & Weightings (Section J5 in the Manual)

-- Period Protections (Section J3)

-- Site Preference (Section J3)

-- Cover-loading, Nominal cover, etc (Section J4)

-- Cover Options, including extra biases, Sixth-form cover rules, half-period cover, etc (Section I)

-- Teacher Role (Appendix 5; on Teacher Details screen)

-- Period Cover Requirements (Section J13)

-- Lesson Cover Requirements, and Subject Cover Requirements

-- Teacher cover limits, 'tend-to-maximum' method, etc. (Section J4, etc)

Most of these can be found in the software on the 'Customize Overview -> Cover Options' page or the 'Customize Overview -> Cover Limits' page.