The UID generator uses a GUID to generate the UID, as descriped in DICOM CP-1156: ftp://medical.nema.org/medical/dicom/final/cp1156_ft.pdf The UID is composed of the following components:
UID ComponentDescription 2.25 The UID root for GUID UIDs as per CP-1156. GUID as Integer The GUID is converted to an integer and displayed in base 10, which can be up to 39 characters long.
The UID generator uses the above components to ensure uniqueness. It simply converts a GUID acquired by a call to Guid.NewGuid into an integer and appends it to the UID for uniqueness.