Recently I re-read some of Lu DongBin's (呂洞賓: the grandmaster of most contemporary schools of the practice) treaties on Neidan (Taoist meditation). I looked into the details of his visualization procedures in key stages of his Neidan process. I found them quite interesting: both imaginative and creative. Perhaps someday I will do a translation of them into English. Many contemporary practitioners of chi-related disciplines still use some forms of visualization (though very limited), like holding a big ball in "embracing a tree" or like drawing chi from a tree through one's hands and fingers when doing stance in front of a big old tree in the park. A dosage of suspension of disbelief will be needed for the contemporary man using visualization, but, I believe, not necessarily needed in the days when Lu's texts was written.
In my practice, I use very little visualization; though I can appreciate the help of visualization to some practitioners, therefore I am not against it.
The use of different visualizations in older texts does boggle many contemporary practitioners of Taoist meditation. In Carl Jung's commentary on "The secret of the Golden Flower", he wrote
According to the Hui Ming Ching the "germinal vesicle" is the "dragon castle at the bottom of the sea." Other synonyms are the "yellow castle", the "heavenly heart", the "terrace of living", the "square inch field of the square foot house", the "purple hall of the city of jade", the "dark pass", the "space of former heaven". It is also called the "boundary region of the snow mountains", the "primordial pass", the "kingdom of greatest joy", the "boundless country", the "altar upon which consciousness and life are made."
Why so many synonyms? Unknown to Jung, they belonged to different visualization systems of different practitioners, mentioned in their respective texts. The reason why Liu HuaYang mentioned them in Hui Ming Jing was that he wanted his readers to know what these symbols meant when they read their authors' respective texts. Actually the Wu-Liu sect didn't like to use too much visualization in their practice.
Having said that and as I mentioned previously sometimes a clever usage of visualization can help some practitioners (not to mention the fact that some visualizations were quite creative and literary). The seminal text Can Tong Qi (周易參同契) is an example. It is rich in imagery and literary allusions. One can use (or waste?) a whole lot of time and energy in fitting the pieces, hopefully, into meaningful visualization tools.
As I mentioned in some previous posts, it is unfortunate that some contemporary Taoist meditation (or Neidan) practitioners wrongly believe that there are some hidden profound meanings under the symbolic texts (whose primary objective were in fact for visualization), and perhaps with a successful deciphering, one can indeed become a true worldly Immortal! I came across some over the internet, and some face to face. And they were not interested in listening to what I said. And I politely leave them with their pursuit.