SASA stands for "solvent accessible surface area]. In a Foldit binder puzzle, a change in SASA can be used to determine how tightly a designed protein "binder" fits a known protein target. SASA is a binder metric.
SASA is a geometric area, calculated in square Angstroms (Å2). In the Foldit objectives area, the symbol "A" is used, perhaps due to technical limitations.
In a binder puzzle, SASA represents the size of the interface between the binder and the target. This is the area that water can't reach. The bigger the area, the better the binding. The term "interface SASA" may be used to indicate that it's not the overall surface area of the protein.
As described in the Foldit blog post Analysis of Protein Binder Designs, SASA is calculated by comparing surface area of the binder and target in the designed "bound" configuration to their separate areas when far apart. The difference in surface area is the interface SASA reported in the objectives area.
Although "solvent accessible" is part of the name, the SASA binder metric is actually measuring the area which will be made inaccessible to solvent (mainly water) when the binder is close to the target.
For example, the target and binder might have an overall surface area of 600 Å2 looked at separately, but only 450 Å2 in the "bound" position. The difference of 150 Å2 is the size of the interface, the interface SASA.
Hovering over a calculated SASA value shows a suggested target value. The initial puzzles with a SASA objective have used a target of 1500 Å2. The larger the SASA score, the better the binding.
As of its initial release in August 2020, interface SASA in Foldit is very slow to calculate. A binder puzzle may have a "SASA score" objective, but players must click "run" or "run all" to see its value. There is no "enable" checkbox, meaning the SASA score won't be calculated automatically with the other scores. At this stage, the SASA score is for information only, and does not affect the Foldit score.
The Foldit developers are planning a feature which would allow calculating SASA asynchronously, meaning without affecting interface gameplay. The initial version of the SASA objective causes a delay of several seconds while calculating, effectively causing Foldit to freeze.