What is the 100 rule in Spain?

Could you possibly explain what is known as the "100 rule" in Spain? I've heard it mentioned in relation to finance or cryptocurrency, but I'm not quite clear on its specifics. Could you elaborate on its origins, how it works, and any potential implications it might have for investors or individuals doing business in Spain? Also, is it a widely practiced rule or just something specific to certain sectors? Thank you for clarifying this for me.