This command is found at
-> .You have two operating methods to stroke a selection, using or without using a paint tool:
Stroke
You don't use any paint tool, but many many parameters are available:
You can set the path width using the slider or the text box. Default is pixels, but you can choose another unit by the arrow button.
This option is not useful for selections that are always closed.
You can choose the shape the selection corners will have by clicking on Miter, Round or Bevel.
This option has not, with selections, the interest it has with paths.
Dash is shown at a pixel level in a small box. You can modify it as you want. An area with thin vertical lines inside the box shows the limits of the pixels. If you click on it you add a pixel to the dash. If you click outside, you remove a pixel.
You can select a dash pattern in the drop-down combination box.
Oblique or rounded strokes may have stepped borders. This option smoothes them.
You can choose either the Solid or the Pattern style. Pattern is that selected in the Toolbox.
Stroke using a Paint Tool
You can select a paint tool in the drop-down box to enjoy all its options in order to stroke the path.