Autosizing: If you always want to use a certain scale, then you would set the exact scale to be used and let Tekla Structures automatically select the drawing size that suits the selected scale.

You can define different scales for main views and section views. All main views in a drawing automatically use the same scale unless you fine-tune individual views manually.

Autoscaling: If you always need to use a certain drawing size, for example, A3, A4 or A1, then you would set the size and let Tekla Structures automatically select the drawing view scale that suits the selected size.

Tekla Structures first tries to use the preferred scale for the drawing views, then the alternative scales, and selects the largest possible scale.

If you are not bound to any specific drawing sizes or drawing view scales, you might want to let Tekla Structures take care of both.