IATSE is essentially a big umbrella union. They cover what used to be the Cartoonist Guild and at most of the established studios the Cartoonist Guild represents animation writers. This was set up under the notion that cartoons didn't really have writers back when the Cartoonist Guild was established - they had people who contributed gags and story men who put them in order and created the storyboards. Even though that has changed over the years IATSE maintains the fiction that animation writers are really just the same as the gag men and story men of the old days. There have been attempts to break the writers away - and by most accounts most of the animation writers want their WGA card, in part for the residuals and in part because the IATSE business agents tend to be more on the side of the companies than the workers they supposedly represent - but the NLRB has tended to land on the side of IATSE when the writers have tried to get out of the Cartoonist Guild.