PlacementConstraint 
    class PlacementConstraint
An object representing a constraint on task placement. To learn more, see Task Placement Constraints in the Amazon Elastic Container Service Developer Guide.
Types
Properties
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                  A cluster query language expression to apply to the constraint. You cannot specify an expression if the constraint type is distinctInstance. To learn more, see Cluster Query Language in the Amazon Elastic Container Service Developer Guide.
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                  The type of constraint. Use distinctInstance to ensure that each task in a particular group is running on a different container instance. Use memberOf to restrict the selection to a group of valid candidates.