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Source Type: There are three Delete Records Source using which you can use the Delete Records action i.e.
Object
Model
Variable
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Object: If you select the source type as Object, then select the object from the drop down list
Variable: If you select the source type as Variable, then select the variable from the drop down list
Model: If you select the source type as Model, then select the model from the drop down list
How many records to delete: If you select the source type as Object, then select the radio button to get single or multiple records.
Delete Specific Records: If you want to delete specific records then you can apply filter accordingly, If the checkbox is checked then you will be able to apply custom filter to get records deleted
Key Field: Using key fields you can delete specific records by using their field name
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