For many years, users like you have defined and solved models in Excel workbooks stored on their local PCs, using the Solver add-in included in Microsoft Excel (which was developed by Frontline Systems).  The Solver add-in is installed on your local PC.  Your Excel workbook is opened from disk, and solved "in place" using your PC's own resources (processor and memory).  Neither Microsoft nor Frontline Systems ever "sees" your Excel workbook, unless you provide it separately, for example in technical support.

Frontline's Solver and Risk Solver add-ons operate differently.  Your Google worksheet is stored online, in your Google document library.  When you use the Solver or Risk Solver add-on to solve an optimization model or run a simulation, the actual solution is computed by Frontline's RASON® Server, operating as a "cloud service" on Windows Azure.  For this to happen, it's necessary to transfer a copy of your Google worksheet, over the Internet, from the document library to our RASON Server.  Your model is solved using the RASON Server's resources (processor and memory).  While this entire process is designed to be secure, you should be aware of it -- please read further below.

The Solver and Risk Solver add-ons are offered under the Chrome Web Store's standard End User License Agreement (EULA), which disclaims all warranties and excludes or limits remedies and damages to the maximum extent permitted by law, and specifies that you bear all risk of using these add-ons.  You should only use these add-ons if you are comfortable with the privacy implications of this process.