Answer:
Line releases are defined by an additional line that is generated by the software. The theoretical background of line releases is described in this Knowledge Base article.
For a detailed explanation of the error, see the model file attached to this FAQ. For the line release No. 3 a definition node has to be set: If the software generated an additional virtual node, Surface 2 would be destroyed.
Imagine the following scenario:
You have four surfaces 1, 2, 10, 14 (see Image 02). When you define a line release to Line 3, it will look as seen in Image 03. As Surface 2 would be destroyed by that (as Line 6 is the same for Surface 2 and Surface 14), Line 6 has to start at Node 3 and at the generated node (see Image 04).
This is where you need the definition node. If it is missing, the software shows the error.
