Subworkflow issue

Hi All ,

I have created the ID -0098 Purchase requisition line workflow (treating as subworkflow) and created another one ID -0100 Purchase requisition review workflow .

Then selected the sub workflow -0098 , in the ID-100 as shown below then system is asking the field must be selected as shown the screen shot .

Can you guys explain what does Document Key and field ? What we need to select here ?

Please explain with an example.