This is different from the Default Transfer Syntax defined in Section 10.1 “DICOM Default Transfer Syntax” in PS3.5, which is Implicit VR Little Endian.

Every origin server is required to be able to convert any Data Set it is going to return into the Explicit VR Little Endian Transfer Syntax, regardless of the form in which it originally received or stored the Data Set, except in the cases of when the decompressed Pixel Data is too large to encode in the Explicit VR Little Endian Transfer Syntax or is received in a lossy compressed form. In the case of lossy compressed Pixel Data, the origin server is permitted to return the lossy compressed Transfer Syntax appropriate to the lossy form that was received. In the case of lossless compressed or encapsulated uncompressed Pixel Data that is too large to encode in the Explicit VR Little Endian Transfer Syntax, the origin server is permitted to return any appropriate lossless compression or encapsulated uncompressed Transfer Syntax, not necessarily that in which the image was received, as an alternative to the Explicit VR Little Endian Transfer Syntax.

If transcoding to the Explicit VR Little Endian Transfer Syntax, a VR of UN may be needed for the encoding of Data Elements with explicit VR whose value length exceeds 65534 (2¹⁶-2) (FFFEH, the largest even length unsigned 16 bit number) but which are defined to have a 16 bit explicit VR length field. See Section 6.2.2 in PS3.5 .