Back when I learned T-Splines through all the free webinars, there were always a repeated (and perhaps unwritten) collection of best-practice procedures to follow. One thing I rarely if ever saw the gurus I learned from was the practice of modeling by deleting individual splines to hone fine-detail. T-Splines ALLOWS this (to some extent, IMO)… and certainly the namesake “T-Splines” is in reference to the “T” juncture that the mathematics at its core permits.
Nevertheless, I would personally classify the creation of 3-sided & 5-sided faces as one of those things-you’re-allowed-to-create-but-shouldn’t to avoid headaches down the road.
In your image, you already have that first face where you erased a spline giving a 6-sided face. T-Splines typically tolerates bad geometry as long as it occurs rarely – by that, I mean usually 2-3 faces away from any other modeling goofs. Instead, HERE you’re looking to delete a spline on an adjacent touching face to the first one. Whatever T-Splines calls this, I have no idea as it’s not the way I model and thus stuff I don’t see.
If T-Splines popped up a warning about a “seashell”, all it took was a Google Search with “tsplines seashell” to land on this thread from 2011 where Tom took time to post a video explanation: