| 1 | Requests acknowledgement that the write operation has propagated to
            the standalone mongod or the primary in a replica
            set. This is the default write concern for MongoDB. | 
         
          | 0 | Requests no acknowledgment of the write operation. However, this may
           return information about socket exceptions and networking errors to
           the application. | 
         
          | <integer greater than 1> | Numbers greater than 1 are valid only for replica sets to request
           acknowledgement from specified number of members, including the
           primary. | 
         
          | MongoDB\Driver\WriteConcern::MAJORITY | 
            Requests acknowledgment that write operations have propagated to the
            majority of voting nodes, including the primary, and have been
            written to the on-disk journal for these nodes.
            
            Prior to MongoDB 3.0, this refers to the majority of replica set
            members (not just voting nodes).
            | 
         
          | string | A string value is interpereted as a tag set. Requests acknowledgement
           that the write operations have propagated to a replica set member
           with the specified tag. |