Feature Extraction in Lowe's Sift

In Lowe's Implementation of sift it has a

[image, descriptors, locs] = sift('1.pgm'); where 1.pgm is the image

I have an output of

             descriptors 199x128 double
             image 250x250 unit8
             locs 199x4 double

Question

1.) are the descriptors the feature vectors?

2.) if i want to write those feature vectors to a .txt file then how should i proceed?

3.) what are locs for in lowes code?

Thanks!