The following are some (but not all) bids that must be alerted -

Weak jump responses
Inverted minors
Jacoby style 2NT over a major
Bergen raises
Fourth suit forcing
Splinter bids (below 4 clubs)
Transfer bids (below 4 clubs)
Forced bids in a "pass or correct" sequence
Conventional responses after a multi-two opening
Trial bids
A 2 club or 2 diamond opening that is strong
A 2NT opening or overcall showing 2 suits e.g. both minors
Conventional opening bid higher than 3NT
An opening bid of 1 club or 1 diamond with less than 3 cards in the suit
Responses to Stayman

You do not alert -   
Doubles or redoubles
Cue bids of opponents suit
Stayman 2 club response to partner's 1 NT
Most calls at the 4th level or higher

You should PRE-ALERT any unusual agreements which could surprise your opponents.
e.g. 2 club opening which is weak
An unusual 1NT opening

SYSTEM CLASSIFICATIONS AND SYSTEM CARDS A GUIDE FOR NEW PLAYERS By Peter Busch

Regulations require that each player at the table has an approved system card that opponents can refer to. This is in accordance with the general principle that your opponents are entitled to know everything about your system and agreements that your partner does. Your opponents can refer to your system card and/or they can ask your partner about the agreed meaning of your bid or play. The system card is divided into several areas:

Basic system This will be Standard American for most new players, though it might also be Precision, 2 Over 1, or Acol. There are others but these are quite rare.

Classification by colour: This is a broad classification of your bidding system and only refers to your 1-level opening bids. The options are green, blue, red and yellow.

Green is the most common, and says that all of your 1 level opening bids are natural and promising at least 8 high card points. Suit bids must have at least 3 cards in that suit, but if you have a 4-4-3-2 shape with 3-2 in the minors you may open 1♣.

Blue systems include a strong 1♣ or 1♦ opening. The most common is Precision where the 1♣ opening is strong and forcing and might not have any clubs. Opening bids of 1♥, 1♠ and 1NT are natural but the 1♦ opening can be artificial.

If any of your 1 level openings is artificial, including a short 1♣ (except for 4-4-3-2 precisely) then your system is Red.

As for Yellow systems, don’t even go there! These are known as HUM systems, standing for Highly Unusual Methods, and include things like an opening pass that shows values, or an opening bid at the 1 level which denies an opening hand.

Any of these systems can also have a Brown Sticker. This means the system has particular bidding sequences which the regulators feel might need special defence. They are reasonably uncommon, but an example is where you have an opening bid at the 2 level that shows a two-suited hand but neither suit is known e.g. 2NT shows 5/5 in either the minors or the majors.

Remember that this is only an introductory guide and there are exceptions to the above. If you have any unusual agreements, you should refer to the official system regulations on the ABF web site for a definite answer on your system classification.

(http://www.abfevents.com.au/events/tournregs/ABFSystemRegs14.pdf)