The "Add headers at score ___ (-999.0 throught 999.0)" setting is the value or score at which spam information is appended to an email message. By default, the value is set at -999.0. In essence, this means that every message you receive Cal Poly Pomona's spam service will add the results of its spam analysis to that message (within the header area).
In most cases, there is no reason why you would want to change this setting (-999.0). However, if you prefer not to have the spam results added to your message unless the message is actually considered spam, you may set this value to be the same value as your "consider spam score" setting. This way, no spam information is appended to the header area of your email unless it is considered to be spam.