What does .Metric do?

I have been analyzing codes from a certain project I found and there is one line of code that I do not understand and cannot find any answer on Google. What does .Metric do in line 28? Thank you!

                    %For Figure 1&2  
I1 = rgb2gray(imread('sir1.jpg'));
I2 = rgb2gray(imread('sir2.jpg')); 
                  
subplot(2,1,1)
imshow(I1)
figure;                                                              
subplot(2,1,2);
imshow(I2)
                   %For Figure 3
figure;            
points1 = detectHarrisFeatures(I1); plot(points1);
                   %For Figure 4
figure;            
points2 = detectHarrisFeatures(I2); plot(points2);
[features1,valid_points1] = extractFeatures(I1,points1);
[features2,valid_points2] = extractFeatures(I2,points2);
indexPairs = matchFeatures(features1,features2);

matchedPoints1 = valid_points1(indexPairs(:,1),:);
matchedPoints2 = valid_points2(indexPairs(:,2),:);

                   %For Figure 5
figure;            
showMatchedFeatures(I1,I2,matchedPoints1,matchedPoints2);

                   %For the Results
u=matchedPoints2.Metric-matchedPoints1.Metric;
                 
if abs(u)<=0.04    
    disp("Matched");
else
    disp("Not Matched");
end

Answer 1

[KSSV]

These lines:

matchedPoints1 = valid_points1(indexPairs(:,1),:);
matchedPoints2 = valid_points2(indexPairs(:,2),:);

The result is a structure. It has a fieldname Metric. This is what being used in the line 28.