An expression is a combination of field references, literal values, and operators that, when evaluated, produce a single value.
Pulpstream uses the MVEL language to specify expressions:
So an Incident_Date field defined in the Incident document can be accessed by specifying Incident.Incident_Date
If that field happens to reference a Smart Object record, then you add a dot and another field name to reference a field in that object. For example Incident.Client.Site. References can continue cascading in that manner so, for example, to get (or modify) a risk score for that site, you might reference Incident.Client.Site.Risk_Score.
Of course, in the Pulpstream interface, you never have to remember references of that kind. You just click the magnifier icon next to the expression window, and select the fields you want to reference. The reference you need is then popped into the expression for you. (But now you know what you're seeing in the expression window.)
To write an expression:
To test an expression: