The label in Blogger is like the tag in WordPress. But it's not necessary to be that though. You can use label as anyway that you want to. However, since BRPS is fully based on label query. You must follow some rules in order to get the most related posts.

The following is the guidelines:

• Label you post with Category + Tag If you have a computer topic blog and you have a post about photo editing using GIMP on Windows. You may label with Windows, which is using in Category way. But it's not good enough. You may label with Windows, Howto. It's still not enough, try Windows, Howto, Photo editing. You can try more Windows, Howto, Photo editing, GIMP. Why not one more? Windows, Howto, Photo editing, GIMP, Filter. Now I believe that you got the idea, you can go on your own.
• As of writing, Blogger supports up to 20 labels to one blog posting. It won't cost you if you label with 20 labels, so why not just label as many as possible and get the most related posts. BRPS queries all 20 labels for you, don't waste the ability of BRPS!
  
• When you label a post, Photo editing give you more precise results than Photo, editing. Because the later may also match File, editing.