Throughout the book, we have been talking about PDI metadata, the data that describes the PDI datasets. Metadata includes field names and data types, among other attributes. Inside PDI, metadata not only refers to datasets, but also to other entities. For example, the definition of an input file—name, description, columns--is also considered as metadata.
You usually define the metadata in the configuration windows of the different steps. You do this manually while you are developing or modifying a Transformation in Spoon. This works perfectly when you know exactly how the data looks like—for example, when you are reading a file—or how you want it to be—for example, when you are creating new fields. There are situations where this is not the case, and you don't know the metadata until runtime. This is a kind of situation where metadata injection can help.