I think the e-file also records the MachineID which is specific to the computer the file is generated from. This is not new for 2018. Whatever it is, I am sure we all have one so that IRS can chase down all the bad guys that are filing fraudulent returns. I you are one of the good guys, don't worry about it. 

Each return used to have DCN (Document Control Number) a few years ago that became the Submission ID. https://www.irs.gov/e-file-providers/ero-duties-after-submitting-the-return-to-the-irs