For general rule help see Rule Engine Introduction
The Rule Engine special value framework is really an aide memoire for literals and field names. It presents name-spaced lists of values and translates from a friendly name to an actual value.
In some cases the friendly name is the same as the value, as with Attribute names. In other cases it's a true mapping as in the case of Identity Silo which returns Guid strings.
Special values are derived from classes implementing the SoftwareIDM.PanelModel.Rules.ISpecialValues interface. Therefore the list of classes available may differ depending on installed modules. In some cases (especially with custom classes) you may have special values accessible to rules running in Uplift or Panel Service that are not registered in the web application and so do not appear in the rule helper.
Special values may used as standalone literals, or they may be used as paths on an object property lookup.
special.Attribute.displayName -> "displayName"
special.Identity Silo.MIM: AD -> "34037d17-6e5e-4bf0-afee-032c33cbac04"
Counters.special.History Counter.Import Add -> context.Counters.2
There are over a dozen special value instances representing hundreds of possible values. Enumerating them all is outside the scope of this document. You can insert special values using the rule engine helper, either by expanding the Special Values section to see a listing with dropdowns, or with autocomplete in the Rule Editor.