Why is there a $ prefix in the startWidth argument but not the connectToField or connectFromField arguments?

db.parent_reference.aggregate([
    {$match: {name: "Shannon"}},
    { $graphLookup: {
      from: "parent_reference",
      startWith: "$reports_to",
      connectFromField: "reports_to",
      connectToField: "_id",
      as: "bosses" }
    }])

All of those arguments are referencing field names, so why only startWith value is prefixed with a $?