Problem when Binding XREFs

[asptamer]

I'm trying to bind a set of 5 xrefs to the root drawing (It's easier to work with when it's bound, trust me!) and I'm seing something strange.

First of all, it will only let me bind one of the references (when I closed and reopened the drawing, it wouldnt let me bind even this one.) The other ones are marked as "Needs reloading" and even if I reload them - the note stays. Perhaps this is what preventing me from binding them? When I right click a reference, the option to BIND is disabled (grayed out.) I've seen this before with nested xrefs, but I could still bind them by starting with the parent reference, and here this does not seem to work.

I noticed that some of the references are marked "overlay" and help wasnt very helpful here:
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Unlike blocks and attached xrefs, overlaid xrefs cannot be nested. If another person is currently editing the xref file, the program overlays the most recently saved version.

If the xref you specify is not already overlaid, a new xref is crated, using the name of the referenced file. You are then prompted for an insertion point, scale, and rotation angle, as described for the INSERT command.
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So here's what I'd like some help with:

1) How can I bind all of the references?
2) Why won't the "Needs reloading" symbol disappear?
3) What does "overlay" mean and how is it different from "Attach"?

Below is the screenshot of my XREF palette.

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EDIT:

Ok, I was able to bind the references after opening each one of them individually and saving it (yes, just clicking save button and closing it.) This done, the root drawing allowed me to reload the references and then enabled Bind/Detach options. The xrefs must've been modified after the root file was compiled and saved.

Still would like some comments to point 3) though
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[CarlB]

Still would like some comments to point 3) though

OK since you explained problem so well and posted a solution

I just copied this from AutoCAD help, you'll find some diagrams and more explanation there.
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You can also overlay an xref on your drawing. Unlike an attached xref, an overlaid xref is not included when the drawing is itself attached or overlaid as an xref to another drawing. Overlaid xrefs are designed for data sharing in a network environment. By overlaying an xref, you can see how your drawing relates to the drawings of other groups without changing your drawing by attaching an xref.