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Source Type: There are three Update Records Source using which you can use the Update 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 update: If you select the source type as Object, then select the appropriate option from group to get single or multiple records
Update Specific Records: If you want to update specific records then you can apply filter accordingly, If you select Update Specific Records then you will be able to apply custom filters to get records updated
Key Field: Using key fields you can update specific records by using their field name
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