How to determine what is property and what are possessions in communism

1. Place a thumb tack at point A.
2. Place a thumb tack at point B.
3. Draw a line between the two using a piece of string.
4. Move the line repeatedly by moving the thumb tacks, until the property of those richer than you is "property" and the property you have is "possessions".

Bingo.  There's your line.  You have now discovered how to expropriateviolently rob those on the other side of the line.

Of course, you need to be prepared for poorer people to draw their own lines to accommodate their desires to rob you.

On a more serious note:

Traditionally, and according to Marxist theory, the line dividing what's public and what's private (translation: what's up for grabsrobbery, and what's not) is supposed to be drawn across capital goods vs. consumption goods.

However, as any person who has ever worked or had a business can tell, since any consumption/capital good can be a capital/consumption good depending on the use you give them, the line is most obviously entirely arbitrary and intentionally designed to rob the most wealthy and productive people, who usually buy capital goods to produce more stuff.  Using your car for an independent business of your own?  Sorry, your car now belongs to "the people" (and by "the people", what they really mean is the handful of rulers).

And that mass theft is exactly what happened, every time, in every place and every time communism was attempted.