Here's the context...
AL and NL were two separate leagues, and "Major League Baseball" was really just a joint business venture for marketing, licensing, TV rights.
The AL-NL Agreement (from 1903!) said the OTHER LEAGUE had to approve of any market changes/relocation.
The Giants wanted to build a new stadium in San Jose after the 1989 Earthquake that disrupted the World Series and damaged Candlestick, but the NL-AL agreement says that's a relocation and requires a vote.
But (A) California earthquake reconstruction money had a DEADLINE for the Giants to request funds. (B) Baseball had to focus on the expiring CBA (C) The commissioner just died and they needed to get a new one.
The Giants got the A's to sign a letter stating that San Jose is considered part of the Bay Area Market so no vote was required and they could meet the state's reconstruction funding deadline.
The problem comes in 1999, when baseball merged the AL and NL and drew up a new MLB constitution... and accidentally changed the meaning of the San Jose letter. The new Constitution lists San Jose as exclusive Giants territory. THE EXACT OPPOSITE OF THE LETTER'S INTENTION. The letter meant "San Jose is part of OUR market" and under the old AL-NL agreement, the A's had every right to move to San Jose.
But both teams had new owners who didn't know the history. So the new Giants owner fought the A's proposed move to San Jose, and won in court. The whole thing was a practically a typo.