Tags in Ignition are points of data, and can have static or dynamic values that come from an OPC address, an expression, or a SQL query. They also offer scaling, alarming, and meta information facilities. The different types of tags include: OPC tags, Memory Tag, Expression Tag, SQL Query Tags, and Complex Tags.
Tags in a PLC are very similar but lack many of the meta properties that Ignition tags have. Usually, these PLC tags only have a value, timestamp, and the OPC Server will sometimes add a quality property to them.