Accessing lookup fields and parent fields
Accessing Lookup Fields
The fields that are looked up from a different flow through a lookup field can be accessed to write expressions. This can be done through the dot ( . ) operator on the lookup field in the current form.
Accessing Parent Fields
Child tables have an isolated namespace. Child table columns will be able to access the values from a different column by referring to the id of the column. However, if you need to access the value of fields from the parent form context then the "parent" keyword needs to be used.
Here are more articles about expressions: