ConcurrencyMode is a special facet that can be applied to any primitive Entity Data Model (EDM) type. Possible values are "None", which is the default, and "Fixed".
When used on an EntityType property, ConcurrencyMode specifies that the value of that declared property is used for optimistic concurrency checks. Essentially, declared properties marked with a fixed ConcurrencyMode become part of a ConcurrencyToken.
The following rules apply to ConcurrencyMode:
The property's type MUST be a simple type. It cannot be applied to properties of a ComplexType.
The property MUST be a declared property.