A13 is a dominant chord - it can be thought of as a dominant 7th chord with extensions. The 13 refers to a major 6th played an extra octave above the root.
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The basic A chord is represented on strings 5 and 4; the dominant 7th on string 3; the 9th on string 2; and the 13th on string 1.
Disclaimer: All content is the author’s interpretation of standard music theory, & should be confirmed & validated by your music trainer, tutor or appropriate professional.