Jamieson, Fausset and Brown on Daniel 8:2

2. Shushan--Susa. Though then comparatively insignificant, it was destined to be the capital of Persia after Cyrus' time. Therefore Daniel is transported into it, as being the capital of the kingdom signified by the two-horned ram (Ne 1:1; Es 1:2-5).

Elam--west of Persia proper, east of Babylonia, south of Media. Daniel was not present there personally, but in vision.

Ulai--called in Pliny Euloeus; by the Greeks, Choaspes. Now Kerah, or Karasu. So in Da 10:4 he receives a vision near another river, the Hiddekel. So Ezekiel (Eze 1:1) at the Chebar. Perhaps because synagogues used to be built near rivers, as before praying they washed their hands in the water [Rosenmuller], (Psa 137:1).