A feature where we can manually add substitutions. For example, I like to use em dashes in my writing (—) but it's difficult to do on my regular keyboard, so I used the substitute feature in the other docs I use to automatically substitute "--" to "—" and use it with other things, too, like adding accented letters automatically onto names (turning eon to éon for example). I know there are keyboards out there that can do it, but I've never been able to find one that works conveniently enough for my writing, and this is a relatively minor but SUPER helpful feature.