Function unit_groups 

pub fn unit_groups(
    document: &Value,
) -> (BTreeMap<u32, String>, Vec<UnitProblem>)

Which port GROUP each symbol unit binds its pins into, by name, and everything about the mapping that does not hold.

The rule is ports::unit_map’s. The one thing added here is the empty part: a document that declares no ports at all maps unit 1 to the DEFAULT_PORT_NAME group, which the first new pin creates. That is the KiCad import’s case and the reason importing a symbol yields ports.