the definition of DSU/CSU

nzq90

I dont understand the following paragraph, can anybody help me:

At data link layer, both X.25 and Frame Relay define a logical DTE and DCE. In this case, the customer premises equipment (CPE), such as a router and CSU/DSU, is the logical DTE, and the service provider equipment (the Frame Relay switch and CSU/DSU) is the DCE.

according these statements, it seems that the CSU/DSU can be DTE as well as DCE! but according to ICND course book says:
DTE refers to the device that looks for clocking from the device on the other end of the cable on a link. The DCE supplies that clocking. For example, the computer is typically the DTE, .and the modem or CSU/DSU is the DCE. which one is correct?
TIA!