[Ur] Question about the Sum metaprogramming example

[David Snider]

"Ur's support for analysis of types is based around extensible records, or
row types. In the definition of the sum function, we see the type parameter
fs assigned the kind {Unit}, which stands for records of types of kind
Unit. The Unit kind has only one inhabitant, (). The kind Type is for
"normal" types."

Could you elaborate on why you used the kind Unit instead of Type?