First of all, this applies specifically to approaching a person. This may be the initial approach, or you might have made plans for the first time and you find her sitting at a table. The left column is the formal zone, which makes both people feel uncomfortable. The middle is the respectfully assertive zone, which is where you'll want to be. The right zone is the threatening zone, which is likely to scare a person unless you know them very well.
Notice how a certain position can be taken (and intended) as either formal or confrontational. Under confrontational, I've also marked the positions that are threatening to a person right off the bat. You can see that they'll make the person feel cornered.
NOTE: In the bottom row, middle column, the bottom-right position is fine on a picnic table, but not on a booth where you'd box the person in. The top position represents pulling up a chair to the booth.