A data source is a container that holds the data you upload to Analytics. Data sources control how uploaded data gets joined with existing data. You configure sources at the property level. You can upload data via the same data source multiple times.
To manage all the data sources for a property, click Admin > (Property) > Data Import.
A data-source type corresponds to the specific type of data you want to import. For example, there are data-source types for cost data, item data, and user data. For each one, you have different options for the dimensions and metrics (the schema) you can upload into.
When you create a source, you define a schema, which is the structure that joins the data you upload with the existing event data. A simple schema consists of a key dimension (the "key") and the import dimension(s) or metric(s). To import data, Analytics looks for key values in event data that match key values in the uploaded data. When Analytics finds a match, it adds (or replaces if data was already collected) the additional dimension and metric values associated with that key to the existing event data. Some data-source types let you use multiple dimensions to define the key, and most can use multiple dimensions/metrics for the import fields.
Key and import dimensions
You can use some of the available dimensions and metrics as your import key and targets, but not all. You’ll see the full list of the available dimensions and metrics in the user interface when you create your data source.
The key is composed of at least one Analytics dimension, and can be composed of up to three.
The key dimensions will vary based on the type of data set you choose. A list of available key dimensions is provided in the drop-down menu in the data-source schema builder.