Complex sentence: A later version of RM/T is described in Date (1983)-we shall call it RM/D in this article-in which Date and Codd improved and refined RM/T, adding an entity type called "designative".
Split into simpler sentences: A later version of RM/T is described in Date (1983); we shall call this the RM/D model. According to Date, he and Codd later improved and refined RM/T, adding yet another entity type called "designative".