X.25 DCE and DTE
An X.25 DTE is a terminal or host computer of some type connected to the X.25 Network.
The Network provides the means for these DTE to communicate with each other.
In the context of X.25 Layers 2 and 3, an X.25 DCE is the local network node to which the DTE is connected.
The X.25 protocol defines the rules for the communication between the DTE and the DCE - communication within the Network may be by some other mechanism entirely.
Note that the DTE/DCE assignment is independent at Layers 1, 2 and 3. When connecting to an X.25 network, then a terminal is typically configured as a DTE at all three levels.
A DXE is a DTE in an X.25 point-to-point configuration that can configure itself automatically to be DCE at Layer 3.