Ncbi Blast Two Parameters'S Question

Hello, I don't know NCBI BLAST two parameters, here they are:

 -num_descriptions <Integer, >=0>
   Number of database sequences to show one-line descriptions for
   Default = `500'
 -num_alignments <Integer, >=0>
   Number of database sequences to show alignments for
   Default = `250'

I want my every reads to generate only one best hit, I have tried -num_alignments, and is seems it works, but I don't know what's difference between -num_descriptions and -num_alignments? Does anyone know and kindly tell me.

Thanks