求助#pragma omp target data和#pragma omp target之间的区别在哪？

/* Create device buffers for a, b, c and transfer data from Host -> Device for a,b */
#pragma omp target data map(to:a[0:size], b[0:size]) map(from:c[0:size])
{
   /* Existing device buffers are used and no data is transferred here */
   #pragma omp target
   {
       int i;
       #pragma omp parallel for
       for (i = 0; i < size; i++)
           c[i] += a[i] + b[i];
   }
} /* Device -> Host data transfer of buffer c is done here*/

请问，红色部分加了data和不加data有什么区别？如果直接把红色这一句替换掉绿色这一句有什么区别？