How can EmEditor display as if it were a tabbed window?

Select the Enable Tabs command so that it is checked.

How do I make the EmEditor window stay open when I close the last open document?

Go to Tools | Customize…, select the Window page, and check the Do not close last document by selecting Close command check box.

What is the split window for?

Sometimes it is convenient when you are writing at the bottom of the text, to view the top of the text at the same time. It is designed for viewing different locations of the same document. To view different documents at the same time, drag the tab outside the window to create a new group and arrange the windows to view both documents.