The scale panel has options for changing the scale along the X, Y, and Z axes separately. You can use this scale feature to change the scale of an XRef along different axes separately. If you want to change the scale of the overall XRef uniformly, then click the checkbox underneath the axes fields and then specify the uniform scale for the entire XRef:
Figure 9.6: The scale panel on the Attach External References window
If you don’t want to specify the scale here and want to specify the value in the drawing, then click the Specify On-screen checkbox and the prompt for the XRef scale will show up in the drawing area while attaching the XRef.