Fixing: It is an agreement between participants on the same side of a market to buy or sell precious metals at a fixed price. The other purpose of fixing could be to maintain market conditions in order to let the price stay at a given level.
Bullion: Bullion is gold, silver or any other precious metal in the form of coins or bars. The word bullion comes from Louis XIII, Claude de Bullion.
There are different participants in this process of Bullion fixing.
The lead participant begins the fixing process.
He proposes a price near the current gold spot price.
The other participants then simulate the result of trading at that price. This is done by each bank. The bank looks at its limit orders to determine how many are eligible to trade at that price.
This value when provided is checked against the overall net amount. If the overall net amount is equal to zero, then all transactions succeed and the fix is complete. Otherwise, the chair changes the proposed price and the process repeats until a fix is found.