As per my understanding, CUID is a 35 characters and normally starts with A, B, C, F, K and M. To know formatting of different ID types, you can refer to this:
SAP Note: 1285103: What are the different types of IDs used in the BusinessObjects Enterprise repository?
CUID: CUIDs are Globally Unique Identifiers that uniquely identify an InfoObject, both within a single CMS cluster and across multiple CMS clusters. Because CUIDs are strings they are less efficient to use and slower to query for. Unlike an object’s ID, an object’s CUID stays the same when it is migrated to a new cluster. BIAR engine and Import Wizard always refer to CUID. When exporting an object to a new system (e.g. from development to production), the CUID stays the same, therefore you can hardcode a CUID (for example in an OpenDocument link).
As mentioned in snapshot, you can check CUID of an object in CMC.