I assume you are reading these to terms from the on-line Help under Drawing Units Dialog:
Insertion Scale
Controls the scale for blocks and drawings that are inserted into the current drawing.
If you insert a block or a drawing that is created with units that are different from the units used in the current drawing, the insertion scale value corrects the mismatch. If you do not want the block or drawing to be scaled, specify Unitless.
Note: If the Insertion Scale is set to Unitless in either the source block or the target drawing, the Source Content Units and Target Drawing Units settings are referenced to determine the scaling ratio. These settings are located in the Options dialog, User Preferences tab.
So in this case the "source" is the drawing you are selecting to Insert into the current drawing. The "target" is the drawing you are currently in.
Hope this helps.
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