I think it has something to do with automatic transmission diagnostics, a reader plugs into it.
I remember reading about it, my '92 has one.
WW 
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Rides:
'08 Chevrolet Avalanche LT3,
'02 LexusSC430,
'94 6-speed LS Coupe
'94 6-speed GS (formally 4cruizn),
'72 Buick Skylark Custom convert., frame-off resto.
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