The alt-shift-l command adds an empty local comment line above the current line in the text editor.

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

After the empty local comment 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 layout command.
Adding a layout command after pressing alt-shift-l.