This is important only for displaying the dashed lines on the screen. In printing lines with "Scaling" both "device" and "device width" will look the same. Select drawn dashed line and in the properties once select it as "device" and second time as the "device width". Try zooming this lines and see how they look. Lines with selected "device width" property is more similar to that which we see in print.
TC Pro 2016, TC Pro 2018, TC Plat 2019
Laptop Asus i7 6500U, dual-core 2,50GHz, NVIDIA GeForce GTX 950M, RAM 12 GB, SSD 480GB
Windows 10 64 bit