[{propertyRating:4.0},{propertyRating:"SC"},{propertyRating:3.0},{propertyRating:5.0}]

db.collection.aggregate([{/** set `propertyRatingSort` to -1.0 when it's "SC"* otherwise use the value as-is*/$set:{propertyRatingSort:{$cond:[{$eq:["$propertyRating","SC"]},-1.0,"$propertyRating"]}}},/** sort descending*/{$sort:{propertyRatingSort:-1}},/** remove the extra field*/{$unset:"propertyRatingSort"}])