There are two types of conditions n rules: predefined conditions and the conditions based on regular expressions.

The conditions of first type are implemented internally and work according to their descriptions:

EMPTY_BODY - Message doesn't contain non-empty text or attachments
NOT_TO_ME - The lists of recipients (To: and CC: header fields) don't contain the address of destination mailbox.
DATE_IN_FUTURE - The message creation date is in future (more than 25 hours after receive date).
FORGED_NDR - Message contains an invalid non-delivery report (NDR). This condition detects reverse NDR attacks.
CAPITAL_LETTERS_IN_SUBJECT - More than 80% of capital letters in subject, which is at least 10 symbols in size

The second type of conditions uses PCRE regular expressions applied to the body of a message or to one of its headers. A condition works if it is possible to apply its expression to the specified part of the message. The check is case-insensitive. If you don't want to study the syntax of regular expressions, enter your expressions as text from messages, digits and spaces. The condition will be met if the text from an expression is a substring of the selected part of message.

Here are several examples of expressions:

·          "foo bar"
The expression is matched if the text contains the substring "foo bar".

·          "foo\s+bar"
The same as above, but there may be one or more spaces between foo and bar.
"^foo"
The expression is matched if the text starts from "foo".

·          "bar$"
The expression is matched if the text ends with "bar".

·          "^foo bar$"
The expression is is matched if the text is equal to "foo bar".

 

Here is the more detailed article on using the rules:
http://antispamsniper.com/art-rules.html

A condition can be marked as Strong. In this case, the rule will be applied if that condition is true no matter whether other conditions of the rule are true.

The negation flag (the Not operation) is applied to a condition if you select the corresponding checkbox.