Pixel Aid allows seeing various statistics for a part of the image (or the whole image), like Median, Standard Deviation and etc. It works in two modes - Bitmap or FITS.
FITS mode requires loaded FITS / XISF file and the Preview Mode to be “On 1:1”. The data is in ADU.
Bitmap mode is used for the other image types or when Preview Mode is other. In this mode the statistics is calculated for the screen black & white data.
The Pixel Aid use the common aid behavior in APT- dragging the window and placing the Source frame over the image part that you want to use.
Source
It is a "transparent" window that shows the image under the Pixel Aid and defines what is the source to be used in the Aid.
The calculated statistics for the data in the Source.
In brackets after the values Min/Max is the number for the pixels with this ADU level. When Whole Image option is enabled the number will be in whole image context, when unchecked displayed is only the number of the pixels in the framed area.
Has effect only in FITS mode. When selected Pixel Aid shows the ADU value one pixel rather than calculating statistical data for Source image. In this mode there is a cross to show the inspected pixel.
The Pixel Aid is accessible from 
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