Interactice Sorting confusion!

Well I have a data set with the following fields

1) Territory

2) Product Category

3) Sub-Product Category

4) Total Sales

I created a report that is a List with a Nested Table

The List have the Territory Field (and have a grouping on Territory)

And the nested Table Have the other 3 fields Grouped By Product Category

The table group header have the product category fields

And the table details have the sub-category and Total Sales fields

Now to the problem, I have no problem adding interactive sort on the two fields in the table details rows.

But I cannot add an interactive sort to Territory Field in the list or the Product Category in the table group header.

Is this a limitation in interactive sort, they dont sort on fields used for grouping other fields !!!

I also couldn't really understand the interactive field options, the help doesn't really explain them clearly

What does the Options

1- Data Region or Grouping to Sort

2- Evaluate Sort Expression in this scope

How are they really used I dont get it? I tried many options hoping something will work for the territory or product category fields mentioned above, but nothing work!

I hope I was clear

I will try to be more precise I have the following grouping

Territory | Category | Sub Category | Sales

USA

Bike

Bike Mountain 100

Bike Racing 200

Toy

Toy Barbie 50

Toy He-Man 74

UK

Bike

Bike Mountain 100

Bike Racing 200

Toy

Toy Barbie 50

Toy He-Man 74

UAE

Bike

Bike Mountain 100

Bike Racing 200

Toy

Toy Barbie 50

Toy He-Man 74

Terriroty is actually in a text field in the list, where I have a nested table with the other fields group on category in the table group header

I have easily add interactive sort to the subcategory and sales table files, using the default values for the options

I cannot field the correct option combination to add interactive sort for the territory or category fields