Re: Altium Underline Tool
It's just a visual aide for schematics that allows you to hightlight different nets in different colors on the sheet currently shown in the workspace. Just click the button, and then click on a net to hightlight it.
You can change the color of the highlight by hitting the spacebar after you push the button. The color will show in the dashed bar on the button.
You can mask everything except one color by using "View>>Underlined Connections", and you can clear all the underlines by using the "Clear" button in the lower right of the workspace.