Each Tag has multiple Parameter types.  The typical parameter that contains the current value is Value.  Value and Alarm Limit Parameters can be a fixed value, come from another local or remote Tag Parameter, come from an OPC Item value, or be a result of a Calculation comprising of multiple local and remote Tags.  When a Parameter is a fixed value it can be set by a local or remote OPC Client, set from an HMI component or programmatic data component, set from OPC Recipe.NET from a database, or OPC Excel using OPC Excel.NET.  When a value Parameter is defined as an OPC Item all writes to it will then be executed to the OPC Server as well.