It's a community update. See the community comes together and makes hats, weapons, taunts, unusual effects, map assets and maps with those assets. And in Frontline's case, it's based on the World Wars Period of time. The period between 1914-1918 and 1939-1945, the period of World War I and World War II. Airplanes, Zeppelins, Trench Warfare, Beach Invasions, Imagine Saving Private Ryan as a Comedy. That's Frontline.

there is a video explaining it, on the front page of reddit, /r/tf2

I'm somewhat confused about this as well. I understand that this is content for the game made by the community, but how does it function within the game itself?
Can it only be used in community servers that host it? Will it be presented to valve and request it be put in the base game?
I'm asking because I have absolutely no clue how these community updates work.

引用自 Twisted_omeN #TeamPlayer：

I'm somewhat confused about this as well. I understand that this is content for the game made by the community, but how does it function within the game itself?
Can it only be used in community servers that host it? Will it be presented to valve and request it be put in the base game?
I'm asking because I have absolutely no clue how these community updates work.

Basically a bunch of people make an update based around a theme and if the community begs enough TF Team will endorse it as a full on update because they're to lazy to make their own.

Problem is while this is happening other parts of the community are also working on a Mayaan update and MVM update and a Mediviel update so the community is all over the place really.

引用自 Twisted_omeN：

I'm somewhat confused about this as well. I understand that this is content for the game made by the community, but how does it function within the game itself?
Can it only be used in community servers that host it? Will it be presented to valve and request it be put in the base game?
I'm asking because I have absolutely no clue how these community updates work.

If community begs enough or Valve thinks it's cool then it may become a real update

ok so basically it was a war themed update with a sfm and everything however end of the line beat it but slowly frontline items are getting in