The "current" DOP network is the network the shelf tools put new nodes into. The user can change this using the simulation menu in the bottom right corner of the main window.
This is always a writeable dopnetwork, so not necessarily point to the network used for the blue cache line display. Instead, it will point to the last current dopnetwork that was also writeable.
If you're building a shelf tool that creates simulation nodes, you should use this function to know where to put them.
Call hou.setCurrentDopNet() to set the current simulation network programmatically.