A and B are already present in the plant. Gene A specifies production of a protein which is used to finish building a seed. Gene B is a switch gene which can turn on gene A. Gene A is turned off in most of the plant most of the time. It is turned on, by gene B, only in the developing seed and only at the end of development.
The other five genes are to be put into the plant. Gene C specifies a protein that will kill the seed. Gene D is another exact copy of gene B, the switch gene, but controlling gene C instead of gene A. It therefore turns on gene C only in the developing seed and only at the end of its development.
Together, genes C and D would achieve the desired function of causing the plant to have only sterile seeds. But this would give the seed company a problem. How can the company reproduce the seed to have a supply to sell?
The other three genes solve that problem. Gene E is another control gene, whose function is to interfere with gene D. As long as gene E is present, gene D cannot turn on gene C and the plant will make fertile seeds.
Gene F specifies a protein which is able to snip gene E out of the plant's DNA. Once gene E is gone it is gone forever and the plant will make infertile seeds. Finally, gene G is a control gene which will only turn on gene F when the completed and fertile seed is exposed to teracycline.
To recapitulate, the seed company would reproduce the plant in the usual way. Gene E would have turned off gene D which is needed to turn on gene C and make the seed sterile. When the seed company was ready to sell seeds to a farmer, they would soak the seeds in tetracycline which would stimulate gene G to turn on gene F, which would cause gene E to be snipped out of the DNA. With gene E gone, when the plant grown from the treated seed makes seeds of its own, gene D would cause gene C to make the protein that kills the new seeds just as they complete their development.
This whole system is much more complicated than anything accomplished by genetic engineers in the present generation of transgenic plants. These have, at most, one control gene and one protein making gene.