What is the 20% rule in Texas?

Could you elaborate on the so-called "20% rule" in Texas? Is this a legal mandate or an industry standard? Does it apply to a specific sector, such as real estate, finance, or perhaps cryptocurrency? How does it impact those who operate within its jurisdiction? Is there any flexibility or exemptions to this rule? And finally, what are the potential consequences for those who fail to adhere to this 20% rule in Texas?