My understanding is that it is a question of how much assistance you need for daily living. Stage one the symptoms are minor, only affecting one side of the body (i.e. tremor in one hand, stiffness on one side of the body). Stage 2 means both sides of the body are affected, but you can take care of yourself and don't need much outside assistance. Stage 3 means you may be falling , or unable to do some things like driving or dealing with finances, but you still able to mostly take care of yourself. You may need help in taking meds for example, but can still make yourself a meal. Stage 4 you need help with every day functions: standing up, getting to a bathroom, cooking, etc. Stage 5 you need help on almost everything.

https://www.parkinson.org/understanding-parkins...