How do I use a story key?

[Else]

What are story keys? how do I use them?

[Miq]

Original story mode in some endings gave a key to access some scenes, now those accesses are  mostly gained from wishpoint shop

[Muratus del Mur]

keys might also be implemented in A25 quests, in general the info you need most is the one you receive when obtaining such a key, because it will tell you what it is good for.