Proxy graphics/Proxy entities.... what are they?

Hi,
I recently worked on some plan drawings. I came accross some "proxy entities". I was not able to move or copy them.
I finally just exploded them, and was then able to work with them. Now the problem is that every drawing that I worked on cannot be opened by the client company that actually drew them in the first place. Has anybody encountered "proxy entites". Has anybody encounterd this problem when dealing with them. What are they?
TIA,
Dennis
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