Nodemap constraint is a property check in the stage. The parallel execution of the job can be restricted to a particular node or nodes at a stage level with this check.
It provides a drop down list of nodes available and by selecting a particular node or nodes the job executes only on that particular node or nodes.
For example, if the incoming data is being processed in parallel mode on 4 nodes then in the next stage if the nodemap constraint is checked and only 1 node is selected then the entire data is processed in a sequential mode i.e. on that particular node.
In case the incoming data processes on 4 nodes and nodemap constraint is checked and 2 nodes are selected then the data is distributed on these 2 nodes and the job process is carried out on the same.