A Short Synthetic is a short call and long put option with both having the same strike prices and expiration dates.
The term synthetic is in reference to the position having the exact same profile as being short the underlying stock/futures contract outright.
The Max Loss is unlimited as the market rises.
The Max Gain is uncapped as the market falls but limited to the strike price minus the stock price as the stock cannot trade lower than zero.
When to use: When you are bearish on market direction.
A Short Synthetic is just the reverse of the Long Synthetic i.e. this option combination behaves exactly the same way as being short the underlying security. So, if you are very bearish on an asset and want the same characteristics as if you were short the asset then you might want to consider using a Short Synthetic.