You can find the definition here:
$HADOOP_HOME/bin/hadoop
...
elif [ "$COMMAND" = "datanode" ] ; then
CLASS='org.apache.hadoop.hdfs.server.datanode.DataNode'
HADOOP_OPTS="$HADOOP_OPTS $HADOOP_DATANODE_OPTS"
elif [ "$COMMAND" = "fs" ] ; then
CLASS=org.apache.hadoop.fs.FsShell
HADOOP_OPTS="$HADOOP_OPTS $HADOOP_CLIENT_OPTS"
elif [ "$COMMAND" = "dfs" ] ; then
CLASS=org.apache.hadoop.fs.FsShell
HADOOP_OPTS="$HADOOP_OPTS $HADOOP_CLIENT_OPTS"
elif [ "$COMMAND" = "dfsadmin" ] ; then
CLASS=org.apache.hadoop.hdfs.tools.DFSAdmin
HADOOP_OPTS="$HADOOP_OPTS $HADOOP_CLIENT_OPTS"
...
And as you can see in the definition, they are exactly the same.