What does your party affiliation mean? Practically, not ideologically.
When you register to vote you can choose a party (Democrat, Republican, Libertarian, etc). You can also decline to check a box. Technically, that means you are Unenrolled, but those people often refer to themselves as Independents. What does it mean and why does it matter? We’re speaking practically here, not ideologically.
Here is what it means for voting. Everyone gets the same ballot for final general elections, no matter what your party affiliation says. In Massachusetts, your party choice affects which ballot you receive during the primary. If you are registered with the Democrat, Republican, or Libertarian parties you can only vote in those primaries. If you are Unenrolled then when you check in at the desk you tell them which primary ballot you’d like to vote for. Asking for a certain ballot does not change your party affiliation.
Not all elections are affiliated with parties. City council, school board, library trustee, water commissioner, select board, those kinds of municipal elections are nonpartisan, so everyone gets the same ballot for primaries and final elections.
So why does it matter? If you want to have a greater voice in a certain party you usually need to be registered with them. The party platform of the Massachusetts Democrats is decided by delegates at the convention. To be chosen as a delegate you need to be registered as a Democrat. For Democratic candidates to appear on the primary ballots for things like Attorney General or Governor they need to receive 15% of the vote of the delegates at the Democratic Convention. To be chosen as a delegate you need to be registered as a Democrat. The rules of the convention are decided by, you guessed it, registered Democrats.
Can you vote for Democrats without being formally aligned with the party? Sure! But if you want to be a leader in your Democratic Town Committee, have a vote at a convention, or vote for the delegates that go to the convention then you need to register.
You can change your registration by notifying an election official in writing 20 days before an election.
For more info: https://www.sec.state.ma.us/ele/elepar/paridx.htm