A useful feature where the user specifies the quantity to take in an orderbook and callback returns the average price paid for that lot size. For example if the offers in the order book for BTC-USD was the following:

Price | size
11,000 | 1
10,500 | 1
10,000 | 1

If the user specified quantity 3, the callback would return 10,500.

From the Gemini best practices:

Redirect limits

Clients may prompt their users for decisions
as to whether or not to follow a redirect, or
they may follow redirects automatically. If
you write a client which follows redirects
automatically, you should keep the following
issues in mind.

Misconfigured or malicious