As explained on page 37 of the notes, a complete proof of the full classification of orientable Seifert manifolds (Theorem 2.2) is not given in the notes. What is missing is the classification of the manifolds that fiber over $S^2$ with exactly three multiple fibers. The statement here is that these Seifert manifolds are all distinguished by their fundamental groups (which are not cyclic so they are not lens spaces, $S^3$, or $S^1\times S^2$), and their fiberings are unique apart from the exceptions listed in part (d). A proof of this can be found in the reference given, namely Orlik's Springer Lecture Notes volume #291. 

The manifolds of type (d) in Theorem 2.2 are among these manifolds. They are closed manifolds so they are not of type (a) or (b). They are also not of type (e) since they do not contain incompressible tori, as shown earlier in the notes. They are not of type (c) since their fundamental groups are noncyclic as noted above. Manifolds of type (d) have 2-sheeted covering spaces which are lens spaces, so they have finite fundamental group. Sometimes they are called prism manifolds, from a way of constructing them by identifying faces of a prism.