How to work with exclusion and suppression lists?

On the Input screen, first, select your main source Data Extension by dragging it to the Selected Data Sources section.

Second, drag-drop the Data Extension with the records you want to exclude on top of the first Data Extension in the Selected Data Sources section.

In the modal that pops up to define the relationship between Data Extensions, choose option DE1 without DE2, and choose the field on which the Data Extensions relate.

For a detailed example, check out the scenario Exclude Contacts in a Data Extension from a campaign.