I read this code but can't understand something.

gles1-kd-hmi.c
/**
 ** gles1-kd-hmi
 ** Windowed HMI example that uses OpenGL ES 1.X for rendering API.
 ** 
 */
#include <ctype.h>         /* Header file for isdigit */
#include <stdio.h>      /* Header file for fprintf */
#include <stdlib.h>     /* Header file for EXIT_FAILURE, EXIT_SUCCESS, atoi */
#include <string.h>     /* Header file for strncmp */
#include <EGL/egl.h>    /* Header file for EGL */
#include <KD/kd.h>      /* Header file for OpenKODE */
#include <GLES/gl.h>    /* Header file for OpenGL ES 1.X */

#include "error.h"

/* dynamically allocated array where we keep information on windows */
KDint32 *windows = NULL;

/* the value of nwin is the number of entries in the windows array */
unsigned nwin = 0;

/* dynamically allocated array where vertices are kept */
GLshort *points = NULL;

/* number of entries in points used; the size of points is nwin * 16 */
GLint nvtx = 0;

static void add_window(KDWindow *kd_hmi_win, KDWindow *kd_win)
{
        kdSetWindowPropertyiv(kd_win,
        KD_QNX_WINDOWPROPERTY_DELEGATE, (KDint32 *)kd_hmi_win);
}

static void set_border(int wid, KDWindow *kd_win, KDint32 kd_size[2])
{
    GLshort *new_points;        
    KDint32 *new_windows;       
    KDint32 kd_dst_rect[4];     
    KDint32 kd_rect[4];         
    KDint32 vid;                
    KDint32 rc;                 
    unsigned i;                 

   
    rc = kdGetWindowPropertyiv(kd_win,
        KD_QNX_WINDOWPROPERTY_POSITION, kd_dst_rect);
    rc |= kdGetWindowPropertyiv(kd_win,
        KD_WINDOWPROPERTY_SIZE, kd_dst_rect+2);
    if (rc || kd_dst_rect[2] == 0 || kd_dst_rect[3] == 0) {
        return;
    }

    
    if (wid == 0) {
        for (i = 0; i < nwin; i++) {
            if (windows[i] == -1) {
                wid = i+1;
                break;
            }
        }
    }

     if (wid == 0) {
        new_windows = realloc(windows, (nwin + 1) * sizeof(*windows));
        if (new_windows == NULL) {
            return;
        }
        windows = new_windows;
        windows[nwin] = -1;
        wid = ++nwin;
    }

    kdSetWindowPropertyiv(kd_win,
        KD_QNX_WINDOWPROPERTY_DELEGATE_POINTER, &wid);

 
    wid--;
    if (windows[wid] == -1) {
        new_points = realloc(points, (nwin + 1) * 16 * sizeof(*points));
        if (new_points == NULL) {
            return;
        }

        /* We will use one vertex array for all of our rendering */
        points = new_points;
        glVertexPointer(2, GL_SHORT, 0, (const GLvoid *)points);
        glEnableClientState(GL_VERTEX_ARRAY);

        windows[wid] = nvtx;
        nvtx += 16;
    }

  

    kd_rect[0] = kd_dst_rect[0] + 1;
    kd_rect[1] = kd_dst_rect[1] + 1;
    kd_rect[2] = kd_dst_rect[0] + kd_dst_rect[2];
    kd_rect[3] = kd_dst_rect[1] + kd_dst_rect[3];

    vid = windows[wid];

 
    points[vid++] = kd_rect[0];
    points[vid++] = kd_size[1] - kd_rect[3];
    points[vid++] = kd_rect[0];
    points[vid++] = kd_size[1] - kd_rect[1];
    points[vid++] = kd_rect[0];
    points[vid++] = kd_size[1] - kd_rect[1];
    points[vid++] = kd_rect[2];
    points[vid++] = kd_size[1] - kd_rect[1];
    points[vid++] = kd_rect[2];
    points[vid++] = kd_size[1] - kd_rect[1];
    points[vid++] = kd_rect[2];
    points[vid++] = kd_size[1] - kd_rect[3];
    points[vid++] = kd_rect[2];
    points[vid++] = kd_size[1] - kd_rect[3];
    points[vid++] = kd_rect[0];
    points[vid++] = kd_size[1] - kd_rect[3];
}



static void remove_border(int wid)
{
    unsigned i; /* loop counter */

    
    if (wid == 0) {
        return;
    }

    wid--;
    for (i = 0; i < nwin; i++) {
        if (windows[i] > windows[wid]) {
            /* 4 lines, 8 vertices, 16 elements */
            windows[i] -= 16;
        }
    }

    /* Decrease the...