Real ID Act-compliant identification documents are required for, among other things, access to federally regulated commercial aircraft.
Currently Maine driver's licenses are non-compliant because the State doesn't require license applicants to provide proof of US citizenship or lawful admittance into or temporary residence in the USA.
The Act was passed in 2013, but Maine still remains non-compliant. Secretary of State Matt Dunlap has resisted getting with this program, saying the necessary changes would cost $2M-$3m - but compliance by July 2019 might be doable.