The so-called Haeng/Su-je(行守制) system was a method of indicating the "discrepancy" between one"s rank(a) and the grade of one"s post(b). Either the letter "Haeng(行)" or "Su(守)" was added to the title of one"s post(b) to suggest such discrepancy. In China, the San"gwan institution first appeared in the Dang dynasty period, and continued through the Seon"hwa period of the Sung dynasty. But the Haeng/Su-je system employed by the Goryeo government was very much different from its Chinese counterparts, in terms of its nature and its functions.
In Goryeo, with the San"jik("honorary") institution in place, officials were bestowed with either titular "Geom"gyo-jik/檢校職" or "Dong"jeong-jik/同正職" posts, and in cases they were given the honorary titular posts and substantially functional ones both at the same time, either "Haeng" and "Su" letters were added to the front of the latter, in order to differentiate one from the other. The "Haeng" was also used in cases in which the recipient originally had honorary W(a)e-Myeong"bu/外命婦 titles or honorary San"jik posts and were provided with an additional title or post in the same vein. In these cases, the letter "Haeng" was added to the front of the "original" San"jik post or W(a)e-Myeong"bu title.
These Haeng"jik and Su"jik examples appear more frequently in "in-grave" epitaphs and census data, than in the official governmental records of history. The former types of materials tended to document the entire career of the buried person or the lineage of the family head with great details, and also included the honorary and hereditary titles of the father, grandfather and even great-grandfather of the person in question as well.
In the early days of the San"jik-je institution, the number of Su"jik posts" appearances and the number of Haeng"jik posts" appearances were relatively the same. But with the passage of time, the number of the former dropped substantially, and only the Haeng"jik posts continued to appear. There seems to have been a couple of reasons. Since the reign of King Hyeonjong, Su"jik posts for the "3 Elders and 3 Masters("3師3公")," which were different figures from the "ordinary" Su"jik posts of the earlier periods, heavily appeared. And after the Mongols" arrival, Su"jik posts with a new meaning of "associate(副)" appeared as well. So, it seems like in order to avoid confusion with all these new usages of the "Su," the Su"jik posts in general were practically avoided as a whole and were not that much used any longer. If there was a working standard or rule for these Haeng"jik and Su"jik posts, then they would have remained in active usage till the end of the dynasty, but there were none, so they did not.
As we can see, the Haeng"jik and Su"jik posts in Goryeo were originally imported from the Dang and Sung dynasties, where the Haeng"jik and Su"jik posts were used to indicate the discrepancy between the rank and the grade of the title, but in reality it was used in a completely different fashion, for the Goryeo governmental officials.