I asked the CFPB and here is the very unhelpful response that I got:
Thank you for contacting the Bureau of Consumer Financial Protection (BCFP) with your question.
The ULI should consist of your institution’s Legal Entity Identifier (LEI), a Loan/Application ID number, and a two digit check digit. The Loan/Application ID component of the ULI should contain at least one character and follow these requirements:
• May be letters, numerals, or a combination of letters and numerals;
• Must be unique within the financial institution; and
• Must not include any information that could be used to directly identify the applicant or borrower
BCFP HMDA Help