Simple question: I need a line between two previously made objects 'empty' so that it remains attached to these vertices: the line itself can't be moved, but moving the vertices modifies the line. Thanks!!
You can use vertex hooks, example: draw a line mesh (2 vertices).
Then in edit mode, select one vertex, press CTRL+H and select "hook to new object". Here an example for a cube mesh, easier to visualize: it works anyway.
A new "empty" will be created where the vertex is, and the vertex is "hooked" to it (meanwhile a "hook" modifier has been created, for your line mesh, with proper relations to empty object).
Do the same with the other vertex. Now you have the line mesh and two empties. Wherever you move the empties, their respective hooked vertex will follow.
You can also hook to existing objects (empties or others) or bones, choosing other options after CTRL+H. You may need to select them first.
In this case you'll probably have to move the control points to the respective empties origin while in edit mode to set their position:
Select the bone again in Pose Mode, go to Bone header and type viz below Custom Shape in the Display panel. It'll change the bone appearance to the line you've created. Next go to Stretch To constraint settings and adjust the Rest Lenght value to make the line fit between Empties.
Lastly you may parent (Ctrl+P) the bone and second Empty to the first Empty to make them move together.