2d/3d registration

I have tried to run the 2d/3d registration example tourial in my own CT dataset by defalut parameters setting. and it worked fine, got 6 transformation parameters and output result images. But I don’t actually know what the 6 parameters mean. whethere it means I have to photograph patient by useing c-arm in that angle(0 degree and 90 degree) and do the transform to CT in the paper’s coordinate then I get CT and patient aligned, or it means CT’s point coordinate in CT’s coordinate system multiply the transform matrix, then get the patient’s corresponding coordinate in the patien’s coordinate system. Can someone help me figure it out? I am totally confused now after searcing and reading some paper and information on the web. Thank you.