I found this explanation doing a search on the internet:
"It is rather long and pompous, but that's really what it is. Both previous explanations were correct, but a clarification is in order. Supposedly a recibo verde is a kind of standardized receipt for free lance workers to recognize they have been paid by their clients. They were meant for such free lance jobs as lawyers, doctors, repairmen, etc., where there was not a labour contract between the client and the service provider. However, since portuguese labour law are very restrictive, many portuguese employers employ recibos verdes as a more flexible way of hiring their staff. They pretend the employees are not on their payroll, and they pay them every month a certain amount against a recibo verde, as if the workers were free lancers doing occasional jobs. This way they don't have to pay vacations, social security, and most of all they can hire and fire their workers as they please. These are known as the fake recibos verdes. They turn the supposedly most restrictive job market in Europe into the most selvatically liberal one, which explains Portugal's exceptionally low unemployment. Ironically, the portuguese government has been known to set the example many times with this unlawful practice."

Here's what it looks like:

https://pt.wikipedia.org/wiki/Falso_recibo_verde#/media/File...

https://pt.wikipedia.org/wiki/Trabalhador_independente_(Port...
Trabalhador independente é aquele que trabalha por conta própria, e não por conta de outrem; na actividade de prestação de serviços ou na produção e/ou venda de bens. Os trabalhadores independentes, no acto do recebimento de um montante pecuniário pela prestação de serviços, venda e/ou produção de bens, necessitam de preencher os popularmente denominados recibos verdes para efeitos fiscais declarativos. O trabalhador independente deve fazer os seus descontos para a Segurança Social consoante os seus rendimentos, e em alguns casos poderá ter de pagar IVA. Em sede de IRS a categoria dos rendimentos dos trabalhadores independentes é a categoria B.

self employed