Pet accidents are the most common reason people call us, and they are also the area where cleaners are least honest. So here is the straightforward version.
A pet accident doesn't stay on the surface. Liquid goes through the carpet face, through the backing, into the pad, and in bad cases down to the subfloor. Anything that only treats the top of the pile deals with the visible spot and leaves the source sitting underneath, which is exactly why the smell returns as soon as humidity rises. The spot looks fixed for a week, then the room smells again on the first warm day.
Getting it out properly means treating the contamination where it actually is. Depending on how far it went, that ranges from a deep flush of the affected area to lifting the carpet, replacing a section of pad, and sealing the subfloor. That last one is not a cleaning job any more, and any cleaner who tells you a surface pass will fix a saturated pad is selling you a week of relief.
Then there's staining, which is a separate problem from odor. Urine is initially acidic and turns alkaline as it breaks down, and on the way it can strip or permanently alter the dye in the carpet fibers. When that has happened, the discoloration isn't dirt — it's the carpet's actual color now. No amount of cleaning brings it back, and the honest options are dye work, patching, or living with it.
The practical difference matters when you're deciding what to spend. Fresh accidents that are blotted quickly and treated soon after almost always come out completely. Old, repeated accidents in the same spot are usually a mix: the odor can be dealt with, the discoloration often can't.
If you can't wait for a professional, blot with a white cloth and plain water rather than rubbing, and skip the supermarket spot removers. Most leave a sticky residue that turns into a permanent dark spot as it collects soil, and several contain optical brighteners that bleach the fibers. Water and a towel do less damage than nearly anything in a spray bottle.