Double-clicking on the node's title bar will allow you to rename the node, which can lead to confusion when displaying a screencap for others.  But it's a direct way to identify specific nodes and their function in the scheme of complex shader noodling.  When creating a compound node3, you can specifically name the inputs, again to identify what is connected to where.  The 'Vasc level' input determines how prominently the veins will appear from the diffuse map.  Value = 1 shows the blue coloring of the veins while the value = 0 will show no veins on the skin texture—all values in between govern the intensity.   And you are correct, Rhia, the 'Vasc Mix' nod is a Cycles-specific node, although you can rename all the nodes for Firefly use also.