> I don't follow. What are B,C,D here? As I understand it, if they get the GPL source from someplace else, they get all the freedoms of the GPL with that source - what is the mising B,C,D?
But they don't get those freedoms on the same program then. If they are given program P under a proprietery license (program P is a modified version of program Q which I distribute), which includes the code I released under the GPL, and they get the source to program Q under the GPL from somewhere else, it does not mean that they get the freedoms of the GPL for program P (since program P is different from Q).
The missing B, C, D could be anything that is covered by the GPL. If I had to assign them each an important missing freedom, I would note B as: the right to modify program P (as I already pointed out in my example), also I would note C as: the right to the source of program P, and D as: the right to redistribute program P. If B, C, D and are restricted by the proprietary relicense of program P, the receiver will not get those freedoms on P, they will only get them on Q.
Please don't take this the wrong way, but perhaps you are not familiar with the concept of copyleft? I would suggest reading these pages, perhaps they will clarify some things that I am attempting (poorly I guess) to convey: