Partial Copy sandbox

This is used for testing purposes. It contains all the metadata from production but also a sample of its data, which will have been selected randomly. That's why this type is useful for testing the borderline behavior of your implementation.

Partial Copy is often used for user acceptance tests, integration testing, and training. It conveys up to 5 GB of sample data and file storage and can be refreshed every five days.

When you create this kind of sandbox, you need to choose a sandbox template (on the Sandboxes page, click on the Templates tab):

Select all the objects you want to be copied into the Partial Copy sandbox. Considering that Salesforce will copy up to 10K records per object type, up to a total of 5 GB, choose the records almost randomly (there is no way to tell Salesforce which category of records you want to be selected).