The idea here is to properly clean your vehicle without scratching the paint or surface. A good detailer will NEVER cross-contaminate when washing your vehicle. What we mean by cross-contaminate is you would never want to use the same wash mitt you use on your wheels and engine bay and dip it into a bucket full of debris, dirt, and containments and then use that same mitt on your paint. We use 3 buckets to prevent cross contamination of dirt, grime and contaminants. One bucket is designated only for cleaning the wheels. This bucket has a ph balanced soup and its own wash mitt along with wheel woolies for the harder to reach areas.
The rest of the car has 2 buckets with Grit Guards: one bucket is for washing and the other is for rinsing. The washing bucket has specialized soap to not harm your paint with 2 wash mitts. The other bucket is full of clean water used for dipping your wash mitt after you wipe down your car. The Grit guards trap the dirt, grime and contaminants and keeps it off the mitts. We wash the car with separate mitts from top to bottom to further eliminate the possibility of scratching the car. This 3 bucket method is the best way to wash and rinse a vehicle because it drastically reduces the amount of abrasives that can  touch your paint and create less scratches. We treat all our vehicles we detail in this manner.