Execute the best orders (or "at best") will be combined with the best price available in the market, therefore it will be corresponded instantly (if possible).
"At best", orders of execution specifies only the amount of bitcoins that must be sold/bought and, instead of limited orders, can not be specified with price limit. The orders of "best execution" are better if:
- You don't have knowledge of the actual structure of the market price;
- You want to buy or sell a certain amount of bitcoins the as fast as possible.