This is from the mechanical drawing for the XSON6 package in this datasheet (page 13).
What do the gray rectangles represent? Note that they don't have any measurements.
Electronic – How to read this drawing
drawing
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If an offset isn't given from the center-line or some other point, it should be symmetrical about the c.l. (always has been in my experience). So 4.7 mm between the inside of GND3/GND4 would mean they are each 2.35 mm away from the c.l., likewise with the 8.3 mm outer to outer.
The pins appear to correspond to the SD card pinout I looked up. It appears backwards on the footprint because the actual SD terminals would be on the right side, but the contacts lead to the left to connect to the board.
The 0.7 is the distance from the center of the smaller (contact) pads to the edge of the larger (mechanical) pad.
The 1.2 is the width of the larger pad.
Notice the dot both arrows point to.
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You'll often see the indication BSC next to one of the measurements. This means Basic Spacing between Centers and is a nominal value. Could have been given here next to the 1mm (pitch) value, or next to a 6mm value between centers of first and last pad.
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Best Answer
See note 2: Can be visible in some manufacturing processes.
My guess is that there may be some exposed copper in this area.