Umbraco stores identifiers in UDI format for most Umbraco object types. This identifier stores all of the metadata required to retrieve an Umbraco object and is parse-able within text. Example: umb://document/4fed18d8c5e34d5e88cfff3a5b457bf2. UDI's can be used in many of the querying APIs.
UDI is currently not an acronym for something. There is no official definition of what it's short for. Therefore it's called UDI
An Umbraco UDI consists of three parts: the scheme, the type and a GUID Identifier. For example: umb://document/4fed18d8c5e34d5e88cfff3a5b457bf2.
Breaking it down:
The scheme is umb:// - this is always the same and makes it identifiable as an Umbraco UDI
The type is document - so in this is an Umbraco node, but it could also be media, member, etc.
The GUID Id is 4fed18d8c5e34d5e88cfff3a5b457bf2 - this is a GUID (dashes removed) which is randomly generated when the item is being created
You can use UDIs in some of the Querying and Management/Service APIs.
There are 2 types of UDIs: