Broadly speaking, Silicones, Inc.'s RTV products can be separated into two categories.  These are Condensation Cure and Addition Cure.  Each involves two-parts that are combined to  begin the curing process.  Further, Silicones, Inc. identifies its Condensation Cure products in this manner... "GI" Series (tin catalyzed).  Addition Cure silicones are designated as the "P" Series (platinum catalyzed). 

There are characteristics unique to both processes that makes each a better performer under certain circumstances.  For instance, if high temperatures are anticipated, then addition cure silicones (platinum catalyzed P-Series) are typically a better choice.  But for economy, general mold making and prototype applications, condensation cure (tin catalyzed GI-Series) would be preferable.  For more in-depth information, read the explanations below.

GI-Series (Tin Catalyzed)
Condensation cure two-component silicone rubbers are excellent for most general mold making and prototype applications.  The GI rubbers are not sensitive to inhibition, meaning they will cure at room temperature over virtually any surface.  They're easy to mix and de-air, and they cure with only a slight degree of shrinkage.  The speed at which the rubber hardens can be accelerated with special activators.  Finished GI silicone molds are excellent for casting polyester, epoxy, polyurethane, masonry, gypsum and candle wax.

P-Series (Platinum Catalyzed)
Addition cure two-component silicone rubbers offer superior heat resistance and cure with virtually no shrinkage.  Although the P rubbers can be inhibited by tin, sulphur or amines, they can be cured in total confinement, and the cure rate can be dramatically accelerated with heat.  Finished P silicone molds are ideal for casting epoxies, low melting-point metals and polyurethanes.