AutoCAD Mechanical toolset supports two distinct methods of scaling in model space; length scaling and text scaling.
Length scaling controls the ratio between the drawn length and the real length. For example, if you draw with a length scale of 1:2, a line that measures 100 drawing units, when dimensioned, shows 200 units. Unlike the AutoCAD SCALE command, length scaling does not enlarge or shrink the line. It simply changes the ratio between drawing the drawn length and measured length.
Text scaling, enlarges or shrinks annotations. It does nothing to other geometry, it only affects annotations. Most AutoCAD Mechanical toolset scaling commands manipulate text scaling.
The AMSYMSCALE system variable controls the “text scale” of model space. The text height of dimensions drawn with a text scale of 1:1 measures 3.5 units. The text height of dimensions drawn with a text scale of 1:2 measures 7 units (3.5 units X 2). The AMRESCALE command enlarges or shrinks annotations. The scale option within the AM:Standards tab of the Options dialog box calls both AMSYMSCALE and AMRESCALE.
Consider a drawing of a large model that is already annotated, and is too large to be plotted on the paper size it is required to be plotted on. You use the viewport scale to shrink the model to fit the paper in layout. However, this shrinks the annotations as well and the text height is no longer the required 3.5mm. In model space, you use text scaling to enlarge the annotations so that they plot at 3.5mm.
Consider a drawing of a large model that is already annotated, and is too large to be plotted on the paper size it is required to be plotted on. When you create a drawing border in model space, you change the scale within the AMTITLE command so that the model fits within the drawing border. It also applies text scaling to the annotations so that they stay at 3.5mm.
AutoCAD’s annotative scaling serves a similar purpose as text scaling.
AutoCAD Mechanical toolset text scaling automatically recognizes annotations and manipulates them without further input. Furthermore, text scaling predates annotative scaling and hence functions under the assumption that no such thing as annotative scaling exists.