"GAAP" stands for "generally accepted accounting principles," and, well, "non-GAAP" means doing things that are not generally accepted. As you might expect, GAAP is the standard because it puts everyone on a level playing field. But for years, Tesla has used both GAAP and non-GAAP numbers in its reports, and that's what the Securities and Exchange Commission told Tesla to stop doing. The SEC said that Tesla was using prohibited non-GAAP "accounting metrics" (as reported by The Wall Street Journal) that were "individually tailored." The SEC sent Tesla a series of letters describing the issues and now considers the matter resolved, the WSJ says. The SEC has been cracking down on non-GAAP numbers across the board, so don't expect Tesla to revert to its old methods in the future.