What is the 97 rule in Spain?

Could you please elaborate on the so-called "97 rule" in Spain? I'm quite curious to know what it signifies and how it applies in the Spanish context. Could you explain its origin, what it's commonly used for, and perhaps provide an example or two to illustrate its application? Additionally, is it a widely recognized or officially sanctioned rule, or is it more of a cultural or informal practice? Thank you for clarifying this matter for me.