Ratio decidendi
the reason for the decision
Where the phrase comes from
The principle in a judgment that was necessary to the result on those facts. Later courts treat that principle as the holding. The rest of the opinion is not automatically the holding.
What lawyers are pointing at
Pulling the ratio out of an opinion is itself an argument. Two careful readers can disagree about which sentences were necessary. What is not in dispute is the method: start from the facts the court treated as material, then the rule it had to use to reach that order. A vivid paragraph that was not needed for the order is not the ratio, however quotable it is. This page maps the term. It does not decide any case.
The same shape in code
The ratio of an incident is the cause that made the fix necessary. A long timeline can bury it. Write the holding in one sentence that names the defect and the change, and keep the rest labeled as context. Promoting a guess from the chat into the holding is how the next on-call inherits a myth.
A concrete picture
A court decides a notice clause failed because it was printed on the back of a ticket in type the buyer could not read. The unreadable type on a ticket is a candidate for the ratio. A judge's aside about the price of tickets is not.