Explain why we look for the closest tax rate of a fee

app/services/tax_rate_finder.rb

@@ -47,9 +47,32 @@ class TaxRateFinder
     end
   end
 
-  # shipping fees and adjustments created from the admin panel have
-  # taxes set at creation in the included_tax field without relation
-  # to the corresponding TaxRate, so we look for the closest one
+  # There are two cases in which a line item is not associated to a tax rate.
+  #
+  # 1. Shipping fees and adjustments created from the admin panel have taxes set
+  #    at creation in the included_tax field without relation to the
+  #    corresponding TaxRate.
+  # 2. Removing line items from an order doesn't always remove the associated
+  #    enterprise fees. These orphaned fees don't have a line item any more to
+  #    find the item's tax rate.
+  #
+  # In these cases we try to find the used tax rate based on the included tax.
+  # For example, if the included tax is 10% of the adjustment, we look for a tax
+  # rate of 10%. Due to rounding errors, the included tax may be 9.9% of the
+  # adjustment. That's why we call it an approximation of the tax rate and look
+  # for the closest and hopefully find the 10% tax rate.
+  #
+  # This attempt can fail.
+  #
+  # - If an admin created an adjustment with a miscalculated included tax then
+  #   we don't know which tax rate the admin intended to use.
+  # - An admin may also enter included tax that doesn't correspond to any tax
+  #   rate in the system. They may enter a fee of $1.2 with tax of $0.2, but
+  #   that doesn't mean that there is a 20% tax rate in the database.
+  # - The used tax rate may also have been deleted. Maybe the tax law changed.
+  #
+  # In either of these cases, we will find a tax rate that doesn't correspond
+  # to the included tax.
   def find_closest_tax_rates_from_included_tax(amount, included_tax)
     approximation = (included_tax / (amount - included_tax))
     return [] if approximation.infinite? || approximation.zero?