The alt-shift-i command adds an empty interpretation line above the current line in the text editor.

Pressing alt-shift-i will add an empty interpretation line above the current line, provided that the current line contains spines (not a global comment, empty line, or exclusive interpretation line). The added interpretation line will have the same number of spine fields as the current line.

After the empty interpretation line has been added, layout commands can be added (or any other use you have for local comments). Below is an example of adding some text in the score through a layout comment.

adding a clef-change.
Adding a clef change after pressing alt-shift-i.