similar to the pen tool when  you edit mark up.

however a thicker line and colours similar to the highlighter's you can buy (pink/orange etc).

also the colour is transparent.

this is used for marking up 2d sheets, this could be a digital replacement for the manual mark ups done on site by the site foreman, that helps track the install of ducts/pipe/cable tray and allows them to buy more stock in readiness for install.