Go to "Actions" menu (1) of process builder. Click "Open Process Settings" menu item (2) to show proper popup (3).

Select "Data Source Settings" tab (4):

There are several fields:

• Data Source - shows current object which records will be source of data for each particular process instance. This field is not editable at process builder, it can be set only at process creation wizard.
• Name Field - Source object field API name, that will be used to get information about record instead of default field (record name).
• The process will be used for a single record - indicates if multiple instances for different records allowed to start or not.
• Source Record - should appear when checked "The process will be used for a single record". If select record here, the process will be automatically started on it. If leave this field empty, user will be prompted to select record when start a process.

• Additional Fields - shows list of linked objects fields that are used in any place of the process configuration. For example, some connector between steps has a condition which contains a second-level field (5). After saving conditions this field will be added to the list of additional fields automatically (6).