Simply put, spots can be washed out, stains cannot. Stains can come from furniture polish, urine, oil based paints, etc. Even though a stain cannot be cleaned out, sometimes they can be fixed depending on the carpet pie type and other considerations. Here are a few common methods:
If you have long non looped carpet pile and the stain has not soaked deep into the pile the carpet may be clipped. Clipping is when a carpet cleaning professional snips the top of the pile just deep enough to remove the stain. It is important your cleaner not to snip far enough down that it is noticeable when light carries across the carpet.
If the stain is too deep into the carpet or too large to be clipped it may need to be patched. This requires an actual carpet installer or properly trained cleaning professional. You will also need an extra clean piece of carpet. Sometimes if the stain is small enough it can be cut out and a new piece hot glued in its place. Cigarette burns are typically fixed this way. You may want to discuss all the possible solutions with your cleaner. Most professionals will charge extra for these services.