yum repo precedence and autoselection

If I have multiple repositories with different rpms that provide the
same item (ie, Provides: foo), how does yum calculate which rpm is the
one it should autoselect if you have another rpm that "Requires: foo" ?

Example:
 - if you had repo A with rpm aaa-0.1-1.noarch.rpm
 - and repo B containing bbb-0.1-1.noarch.rpm
 - and repo C containing ccc-0.1-1.noarch.rpm
 - and all three rpms had the same Provides: foo defined
 - which rpm gets autoselected if your package list Requires: foo ?

I'm assuming there's some unwritten law of rpms saying "no two rpms
should claim to provide the same thing", but regardless how does yum
figure out what to autoselect under the hood when it parses the various
enabled repositories ?

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