The Seleucid Empire extended further east beyond the boundaries of this map.  At their furthest point they controlled part of India.  One of their kings gave that area up in exchange for elephants to be used in wars with the Ptolemies.  They still controlled the larger part of Parthia, some of which was in the area beyond the right edge of this map.  The area shown is sufficient to give the reader a general idea of the extent of the Seleucid Empire.