Add some explanations in doc regarding the fraction of the R,X,G,B values in dangling lines compared to original line

docs/grid_model/network_subnetwork.md

@@ -339,7 +339,19 @@ Optional:
 - $R$, $X$, $G$ and $B$ correspond to a fraction of the original line and have to be consistent with the declared length of the
   dangling line.
 
+Regarding the values of $R$, $X$, $G$ and $B$, the fraction corresponds to the fraction of length of the dangling line compared to the original line. For example, if we have:
+- a line $D$, with length $L$ and two dangling lines $D_1$ and $D_2$ such as $D = D_1 + D_2$
+- $D_1$ with length $L_1 = k_1L$ with $k_1 = 1/4$
+- $D_2$ with length $L_2 = k_2L$ with $k_2 = 3/4$
+
+Then we will have for the dangling lines respectively:
+- $R_1 = k_1R$, $X_1 = k_1X$, $G_1 = k_1G$, $B_1 = k_1B$
+- $R_2 = k_2R$, $X_2 = k_2X$, $G_2 = k_2G$, $B_2 = k_2B$
+
+Note that if we had only $D_1$ without $D_2$, the previous relation between $D$ and $D_1$ would still hold.
+
 In case the line is a boundary, a pairing key $pairingKey$ (in previous network versions $UcteXnodeCode$) is defined beside the characteristics of the table. It is a key to match two dangling lines and reconstruct the full boundary line for both UCTE or CIM-CGMES formats.
+This key is used for the merge of two dangling lines into a [TieLine](#tie-line).
 
 A dangling line has a `Boundary` object that emulates a terminal located at boundary side. A dangling line is a connectable
 with a single terminal located on the network side, but sometimes we need state variables such as active or reactive powers on