Maybe it is clearer if one notes that a pair of the characters | ISOLATE the associated text – you need two, but you also need the “link adjacent” space character in some instances (which I indicate with * but needs to be selected from the drop down to replace those.) Without the * between Series Title and the word Series (which I assume you want there as text when there is a Series title) the word Series will appear in all instances.
Book Title| (|Series Title*Series|*Volume|)|,*Pages|.