The state abbreviation (such as MO) is sometimes required because quite often there is more than one city with the same name in the US. There is a “Portland” in Maine and one in Oregon; Kansas City, Missouri and Kansas City, Kansas (same city, but the state line runs across the city), and almost each state has it’s own “Springfield”.
It’s the two-letter code of the state.
MO is Missouri.
List of U.S. state abbreviations – Wikipedia
You can learn them with this geographical game of States of USA or text game of states abbreviations.
Credit:stackoverflow.com‘