[AstralStorm]

The difference is owning major assets. Be it a company, many housing investments, office spaces, factories, big farms, major amounts of stocks, financial institutions etc. Secondary, amount of political and medial power and pressure wielded.

Finally, a condition wether your own labour is not technically required for survival.

Most petit bourgeoisie still has to work, does not wield direct (individual) political power nor has big enough assets to survive despite major downturns.

The difference from proletariat is that they can rent and hire labour. Sometimes they get to manage it too.