The script can designate names to refer to certain rules. This name is created using a DefineRule statement.
If you the follow a statement by a word in square brackets, when that statement is actioned, VPOP3 will add the specified word to a list of 'matched conditions' which can be used in reporting.
Also, the DefineRule statement indicates a 'weight' for the rule being defined. This weighting can also be modified via Settings → Spam Filter → General → Rule Weights in VPOP3's Administration Settings.
The weight defines a multiplier for any numeric operations performed by the statements that use it.
Consider the following; if 'myrule' has a weight of 0.5
${Score} += 100 [myrule]
This statement will actually add 50 (100 * 0.5) to the 'Score' variable