In Japanese culture, the term Shingyōsō (真行草) has it’s roots in many sources; This is a concept that can be found in everything from Toshi Keikaku (City, urban, and town planning; 都市計画), to the categorization of utensils found in Sadō (Tea Ceremony; 茶道), it is however most known for its use in Shodō (Calligraphy; 書道). In general this system can be thought of as a measure of formality:
- Shin 真 represents the most formal or symmetrically structured. For city planning this would refer to the cities that have a rectangular perimeters and orthogonal axes, and are regarded as formal cities.  In tea ceremony, when offering tea to a noble, or at a shrine or temple, a matched set of bronze utensils from China is used with a specific utensil stand. These are really (formal) utensils. In the case of calligraphy, this would be recognized for being quite crisp and font-like, often times with sharp serif while other times without any such flourishes. This would be used for more clerical situations such as temple administrative documentation.
- Gyō 行 represents a semi-formal presentation of the art in question. for traditional urban settlements in Japan, gyō would be the castle town. These towns usually have a combination of the other two styles and may be called ‘Gyō‘ cities. The center area in castle towns usually follows the curve of the castle moat, while the perimeter may be more rigid or relaxed.  For Tea ceremony, this would represent utensils between Japanese ceramics and bamboo and that of Chinese steel wear, and as such, is semi-formal.  lastly, for calligraphy, this would be the equivalent to general handwriting found in English, commonly used for daily communication, note-taking, and illustrates a bit of character from the author. 
- Sō 草 is completely informal, and though may not be welcome in many high-etiquette venues, is none the less appreciated for its Wabi-sabi qualities and character. The cities located further inland were developed along the water routes, and respond to the curves found in the rivers and waterways. These cities therefore are
planned in ‘Sō’ style.  In contrast to these Chinese utensils there are ceramics made in Japan that have an earthy flavor and simple utensils made of bamboo and wood that bring out the quality of their materials just as they are. These are ‘Sō‘ (informal) utensils.  Lastly, Cursive script. Also called Grass script. Flowing style, with slender lines, and composed with rapid fluid strokes. This is the type most often used in formal Japanese calligraphy.
Now that we have established the nature of the concept Shingyōsō, we get to the question of “what does this have to do with shibari?”
Well, as one may have been made aware of, many of the ties used in shibari, and many more that are still in development, were inspired, borrowed, or derived from Hojōjutsu, the martial art of rope arresting.
Interestingly, the practices of arresting the opponent with rope presents one of the most clear examples of Shingyōsō available in the martial arts. There are a few ryūha (traditions) in particular that made use of Shingyōsō quite explicitly, such as Ichiden-ryū, Taishō-ryū, Kentoku-ryū, Sasai-ryū, and Hōen-ryū, where many of the ties have variations that increased in complexity as they were considered more formal. Each stage expresses the progression from informal to formal via placement and complexity of the rope on the captive.
Alternatively, for some traditions, the ties were divided up in measure of expected longevity, where Sō was for temporary ties for use with the short rope in immediate capture situations, while Shin was the category for the more permanent ties performed using the longer rope, and of course the Gyō ties were for anything in between that.
In the matters of shibari, it’s a bit more of an aesthetic of “completeness” or “unfinishedness”. Many times in class I’ll be showing a tie, and stop to state, “this is the tie”, and then continue either with the rest of the rope in hand, or with adding more rope. This pause is “sō”, it has done it’s job, but it gives the feeling of being incomplete. This can manifest in the examples from Ipponnawa, or from examples of Kuzushi (untidy, unbalanced; 崩し). The rope covers little and is often times asymmetrical.
Though simple and for some dis-interesting, this level of informality requires the least amount of nit-picking (finite knot tying), as it is oftentimes covering the most space with the least complexity. This allows for greater flourish, flow, and a focus on other things (such as play).
Shin, by contrast, it a sort of complete, well structured, and solid seeming architecture of tying. Here, things are usually pretty symmetrical, and it is usually at this point that some would consider a box tie (for example) to be suspension worthy, though such may not always be the case.
Finally, Gyō is in-between, it is neither highly structured, nor is it loose and relaxed like sō. This sort of level is often suitable for partial-suspensions. Interestingly, one could almost measure shingyōsō by how much rope is used: one rope is sō, two is gyō, and three is Shin, though it would be very shallow to say that this is the case, it almost equates here.
But to delve down that rabbit hole would be to digress. Shingyōsō in shibari is a matter of recognizing a certain degree of formality in the ties and the qualities that are made available thereof. More on the matter can be gleamed from the workshop on this very subject:
 Shintaro Hanazawa, Yukio Nishimura, Takeru Kitazawa, Naota Nakajima. Shin, Gyo, So: The Traditional Concepts of Spatial Design in Japan. (2004)
 Omotesenke Fushin’an Foundation. Shin-gyo-so (formal semi-formal, informal) ranking of utensils. (2005)
 Schumacher, Mark. Shodou – Japanese Calligraphy, Literally “The Way of Writing”. (2013).
 Seiko, Fujita. Zukai Hojojutsu. (1986).