Two Syriac letters, the fourth and the twentieth, share one shape. A dot below the stroke makes it dalath and a dot above makes it rish, and there is no other difference. It is the smallest distinction the alphabet asks anyone to carry, and it separates a d from an r, which in a Semitic language means it separates unrelated roots.
The earliest writing does not make the distinction at all. The point is a later refinement, and manuscripts from before it, or copied by a scribe who did not bother, present a reader with a shape that could be either letter. Modern editors face the same page and have to decide. The decision is usually easy, because the word is known and only one reading yields Syriac, and it is not always easy, and it falls most often on names, on numbers and on words borrowed from other languages, where the context that would settle it is exactly what is missing.
The character standard records the undifferentiated shape separately for this reason, so that an edition can show what the manuscript shows rather than what the editor concluded. That is a small piece of machinery with a large purpose: it lets a reader see where a text is certain and where it has been read.
The same pointing habit spread outward from this pair. Points came to mark the hard and soft readings of the six letters, the plural of a noun, and eventually the vowels themselves. Syriac writing above the line and below it begins with the need to tell a d from an r.