As part of a September settlement, Musk promised to get sign-off from Tesla lawyers for any tweets that "contain, or reasonably could contain" material information—legal jargon for information that's significant for people trading Tesla's stock. The SEC argues that Musk's February tweet, stating that Tesla would produce "around 500k" vehicles in 2019, violated that requirement.
Musk disagrees. He argues that he was merely repeating Tesla's earlier production estimates. And he insists he was entitled to use his own judgment to determine the information was not material—and therefore didn't require pre-approval by Tesla's lawyers.