The institutional feed is designed for clients who are either trading in larger sizes per-click or have a style that doesn't fit the mould for our default price feed so that we can no longer off that feed to them.
There is a larger top-of-book volume available with the same low commissions and swaps that we are known for, but the spreads are naturally a little wider to account for the deeper liquidity. This feed in general will provide less slippage from the quoted top of book price and give you more certainty of the executed price you will receive on your trade when you enter and exit the market.
The feed provides more certainty on the price that can be received for larger trade sizes by offering (usually) 10 lots at top-of-book price on FX Majors and 5 lots at top-of-book on non major FX;