Offset value in OpenCV's face detection implementation [closed]

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When we train OpenCV's face detection CascadeClassifier, we use 24 x 24 images. Then feature rectangles and related left/right leave values are those in xml files extracted from 24 x 24 images.
But when in detection implementation APIs at cascadedetect and detection_based_tracker files, the features are extracted from the rectangles from the re-sized images (re-sized with a scale factor) at line 1014 of cascadedetect.cpp as

if( !featureEvaluator->setImage( image, data.origWinSize ) )
return false;

Then the each pixel (x,y) is run through for the LBP_classifier at line 955 of cascadedetect.cpp as

for( int y = y1; y < y2; y += yStep )
{

    for( int x = 0; x < processingRectSize.width; x += yStep )  
    { }

}

Features are calculated from the rectangles of the re-sized image and offset value for each pixel location is used in the feature calculation.

int c = featureEvaluator(node.featureIdx); //cascadedetect.hpp line 461

#define CALC_SUM_(p0, p1, p2, p3, offset) ((p0)[offset] - (p1)[offset] - (p2)[offset] + (p3)[offset]) //cascadedetect.hpp line 58

My queries are
(1)LBP feature xml is constructed using 24 x 24 images. Why feature rectangles are used from the re-sized images (not from 24 x 24 sub-window areas of the re-sized image)?
(2)What does the offset value do in the following line

#define CALC_SUM_(p0, p1, p2, p3, offset) ((p0)[offset] - (p1)[offset] - (p2)[offset] + (p3)[offset]).

I checked p0 is integer pointer and pointing to an integer value. What does it((p0)[offset]) mean?
Thanks