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 aux layer, the code can work out the net result of all transformations the
 subc suffered since creation.
 
+<h5> 2.4.5.4 Subcircuit uid </h5>
+<p>
+Each subcircuit has a globally unique ID generated (using libminuid) when the
+subcircuit is first created from scratch. The uid field is retrained through
+copies as it is used to identify the original source of a footprint even
+if the footprint had minor modifications. The user shall change the uid if
+a substantial modification is made to a footprint.
+
 <h4 id="padstack"> 2.4.6. Pad stacks </h4>
 <p>
 A pad stack is a generic pin/pad/via object that may contain any combination