Good lord. This isn't hard. And yes they can have secondary meanings, and most likely do, but this is the common usage.
https:// study.com/academy/lesson/what-is-a-map-key-legend-definition-symbols-examples.html
>>Just like your house key opens your front door, a map key or legend >>opens up a map. The map key is what gives you all the information you >>need for a map to make sense.
Map - Overview of area of interest.
Legend - Identity as to the icons on the map and what they mean.
Legend is a Key.
Q went through and explained events in a cryptic manner, providing an overview - a MAP.
Events provide us with a LEGEND, which is a KEY to unlocking the secrets in the in the MAP.
Future proves past.