A variable is an identifier against which a value can be stored for later use.
With respect to the System Architect reporting system:
SELECT fieldname AS target_field
Copy a value to a field for later use.
Last-assigned or last-read values of fields can be used in place of values in WHERE field-identifier operator field-value1[, field-valuen] FIELD clauses.
FIELD { SOURCE FIELD "fieldname" }
Specifies that the last value read from the designated field will be used when this field is selected.
FIELD { SOURCE FIELD "" }
Specifies that the last value assigned to this field will be used when this field is selected.