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To save me or someone else having to figure it out again.

app/assets/javascripts/admin/products/controllers/units_controller.js.coffee

@@ -1,3 +1,4 @@
+# Controller for "New Products" form (spree/admin/products/new)
 angular.module("admin.products")
   .controller "unitsCtrl", ($scope, VariantUnitManager, OptionValueNamer, UnitPrices, PriceParser) ->
     $scope.product = { master: {} }
@@ -12,13 +13,15 @@ angular.module("admin.products")
 
     $scope.variant_unit_options = VariantUnitManager.variantUnitOptions()
 
+    # Extract variant_unit and variant_unit_scale from dropdown variant_unit_with_scale,
+    # and update hidden product fields
     $scope.processVariantUnitWithScale = ->
       if $scope.product.variant_unit_with_scale
-        match = $scope.product.variant_unit_with_scale.match(/^([^_]+)_([\d\.]+)$/)
+        match = $scope.product.variant_unit_with_scale.match(/^([^_]+)_([\d\.]+)$/) # matches string like "weight_1000"
         if match
           $scope.product.variant_unit = match[1]
           $scope.product.variant_unit_scale = parseFloat(match[2])
-        else
+        else # "items"
           $scope.product.variant_unit = $scope.product.variant_unit_with_scale
           $scope.product.variant_unit_scale = null
       else if $scope.product.variant_unit
@@ -32,6 +35,8 @@ angular.module("admin.products")
       else
         $scope.product.variant_unit = $scope.product.variant_unit_scale = null
 
+    # Extract unit_value and unit_description from text field unit_value_with_description,
+    # and update hidden variant fields
     $scope.processUnitValueWithDescription = ->
       if $scope.product.master.hasOwnProperty("unit_value_with_description")
         match = $scope.product.master.unit_value_with_description.match(/^([\d\.,]+(?= *|$)|)( *)(.*)$/)
@@ -45,6 +50,7 @@ angular.module("admin.products")
         value = window.bigDecimal.divide(value, $scope.product.variant_unit_scale, 2) if $scope.product.master.unit_value && $scope.product.variant_unit_scale
         $scope.product.master.unit_value_with_description = value + " " + $scope.product.master.unit_description
 
+    # Calculate unit price based on product price and variant_unit_scale
     $scope.processUnitPrice = ->
       price = $scope.product.price
       scale = $scope.product.variant_unit_scale

app/assets/javascripts/admin/products/services/option_value_namer.js.coffee

@@ -1,4 +1,5 @@
 angular.module("admin.products").factory "OptionValueNamer", (VariantUnitManager) ->
+  # Javascript clone of VariantUnits::OptionValueNamer, for bulk product editing.
   class OptionValueNamer
     constructor: (@variant) ->