Add/Remove Custom Object Field as Smart List/Trigger Constraints add-remove-custom-object-field-as-smart-list-trigger-constraints
Marketo Engage provides fine-grain control over the Salesforce custom object sync. This allows you to select the fields available as constraints in custom object filters and to use them as triggers in Smart Campaigns.
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Click Admin.
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Click Admin and then Salesforce Objects Sync.
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Salesforce Objects Sync appears in the left column.
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Select the object you wish to modify.
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Click Edit Visible Fields.
note tip TIP If the Edit Visible Fields button is grayed out, the object is currently in use in a Smart List or Smart Campaign. Remove all associations to proceed. 
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If your global sync is enabled, click Disable Global Sync.
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Check the boxes next to the filter/trigger constraints needed and click Save.
note note NOTE All fields are selected by default to be constraints on filters. -
Click on the Fields tab to confirm your changes.
note note NOTE Don’t forget to re-enable your global sync!
Now your Smart Lists and Smart Campaigns have even more power.