Sovereigns are also defined by the striking method, how they are produced. There are three methods of production
Proof is a grade which reflects the high standard of finish of the coin. Proof sovereigns were often issued to commemorate new portraits and can be found in both a matt finish (1817 - 1953) and a highly polished mirror and matt finish (1979 - present). Proof sovereigns have been struck between four and seven times to achieve the sharp definition of the design. They will have a mintage limit and will have been issued in a presentation box. You should never remove the coin from the capsule as they are easily damaged which will affect the value and desirability. Between 1982 and 1999, sovereigns were only struck in a proof finish.
Brilliant Uncirculated is a term that has been introduced by The Royal Mint, it is not a grade. It indicates that the sovereign has been struck twice during production to give a slightly sharper definition. However, the prices charged do not usually reflect the intrinsic metal value or value for money. Often the Brilliant Uncirculated coins are issued in a presentation card or box and may have a certificate.
Bullion is not a grade, it is a term that is used to describe coins which are sold at or close to their intrinsic metal value. Sovereigns which were issued for circulation between 1817 - 1932 were minted as uncirculated coins with a face value of one pound. Both circulated and uncirculated sovereigns which are sold at a low premium can be called bullion sovereigns. Uncirculated sovereigns are usually struck once when they are minted and are often bought for investment purposes.