Guide Lines can appear when the Smart Cursor senses element nodes and edges. In this case the Smart Cursor's shape will be a Checkmark or Mercedes sign version.
In case of Library Parts this gives a special case:
When you select a Library Part, you will see selection hotspots. These are the "nodes" for a Library Part.
Also, edges seen in Library Parts are not necessarily sensed by Guide Lines. In GDL there are special commands called HOTLINE2 and HOTARC2 that define lines and arcs, respectively, which will be sensed as lines and arcs when the Library Part is placed.
So if the 2D Script of the given Library Part is not written to include these commands, Guide Lines will not sense an edge in that Library Part no matter how many lines and arcs you see in its 2D Symbol.
If you place a Stair, for example, you will see that almost all its special points is sense as nodes and almost all its lines are sensed as edges by the cursor. But not all Library Parts are written to be this smart.
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