Partial Data sandboxes include all of your organization’s metadata and add a selected amount of your production organization's data that you define using a sandbox template.
A Partial Data sandbox is a Developer Sandbox plus the data you define in a sandbox template.
Sandbox Template is the choice of the objects you want to pull on to the sandbox.
Steps in the process
Make sure you have Partial Copy Sandbox available
Create a Template
Create Sandbox with that template
Wait for the Sampling
Additionally, as defined by your sandbox template, Partial Data sandboxes can include your organization's standard and custom object records, documents, and attachments up to 5GB of data and a maximum of 10,000 records per selected object.
A Partial Data sandbox is smaller than a Full sandbox and has a shorter refresh interval. You can refresh a Partial Data sandbox every five days.