I think Martin described it well… I even double checked it. When going to object mode with multiple objects, with curves and meshes (at least) when you go into edit mode, you can see all that you have of the same type as the active object that is selected. So if a mesh object is the active object selected, all selected mesh objects will have their verts visible, and with a curve object actively selected, all curve points from the curve objects will be visible… but only the one you have actively selected. So if you have both types in your selection and your active object is a mesh, you’ll only see verts of the mesh objects, but not of the curve or grease pencil object.
Now, I was trying this on the 5.1 alpha (from a few weeks ago), so not sure how this is on other versions, but in the version I was testing this on, with Grease Pencil objects, even with multple grease pencil objects selected, it only showed the points of the active grease pencil object. One of those rare instances where things are not the same across all parts of Blender, for whatever reason.
(This ability to select multple objects and be able to simultaneously edit them is a relatively new feature in Blender. and has only been around since about 2.8.)
