I am going to assume you are talking about the editing options - what each type of editor can do that the other one cannot.
The text and diagram editors both work on the same workflow, they are just different ways of editing.
The answer to "what can we do in one and not the other" is actually "nothing of much importance that you'll want to do day to day"
The text editor is much older, and when the diagram editor arrived, it was not able to do a lot of stuff you could do in the text editor, it could only do the absolute basics. But it is the preferred editor and Atlassian quickly added almost everything from the text editor into it. The text editor has been maintained, but the diagram editor has had one thing added to it that the text has not (at least, not that I can find)
The diagram editor lets you add "all -> status" transitions. The text editor can edit existing "all" transitions, but can't create them.
The text editor lets you edit the steps (steps are not status, but do have an exclusive 1:1 relationship with status). The diagram editor hides the steps from you completely
Hello Atlassian Community! Feedback from customers like you has helped us shape and improve Jira Software. As Head of Product, Jira Software, I wanted to take this opportunity to share an update on...