When arriving at a Supercharger (perhaps within 500 ft), a pop-up will show the layout of the specific stalls and show which open stall is recommended to provide the fastest charge. This improves utilization as cars are cycled through faster, and makes for happier users and less chance of having to wait when a location is very busy.
When arriving at a Supercharger with multiple cars already charging, it’s often difficult to figure out which stall will provide the fastest charge, as every two stalls operate on a single supercharger. While many locations are well labeled (1A, 1B, 2A, 2B, etc.), quite a few are not labeled at all or are labeled in a way that makes it very difficult to know the paring (Hawthorne and Gilroy, CA come to mind).
In addition, when most stalls are occupied, you do not know how long or at what charge rate existing cars are at to better choose which open stall to use. For example, at a four stall location, 1A and 2A are in use, and 1B and 2B are open. This means each car has 100% of each Supercharger and are charging at the fastest rate. If car 1A is pulling the maximum power, and 2A is near fully charged, then you should use 2B to provide the fastest charge.
In this example, if the 1A and 2A owners are not nearby, you need to do the Supercharger dance – Try one open stall and wait 2-3 minutes to see what charge rate you’re getting. If it’s fairly low, then you need to disconnect, move to the other stall to see what it’s charge rate is. If slower, then return to the first stall. While many don’t bother with the Supercharger dance, picking the best stall can easily save 30 minutes or more!
(voting combined from a close duplicate that has been removed)