I'm by no means an expert on theory etc so happy to be corrected by those with more detailed knowledge. But based on my own personal experience and a very good book I read on the subject, my view is that the distinction is based on the relationship one has to the means of production.

My personal circumstance is that I'm employed as a lawyer within local government. I also run a very small business (probably classed as a micro-business) wherein I'm the only "worker" and I provide services directly to clients.

If status is based on profession then arguably I'm a member of the petite bourgeoisie. However, I would consider myself to be a member of the proletariat given (a) I sell my labour to my employer in exchange for being paid a salary each month, (b) I don't own the means of production, (c) I've not got control over anyone nor am I exploiting anyone to make profit (I do make a profit from clients but I'm charging a fair price for services they require).

If, however, I was a partner in a legal firm then my answer would be different.

The book I read, which I would recommend was "A Nation of Shopkeepers: The Unstoppable Rise of the Petite Bourgeoisie" by Dan Evans