Thanks for your comments.
Ad.1 Center ID is not necessary because Client can be attached to Group or directly to the Branch. So, I think that it is enough to specify either Group ID or Branch short name.
Ad.2 I think that every user may want to have his own convetion so it will be better to not specify any convetion and accept ever file name.
Ad.3 I like this idea. It is more convenient to use "ref" sheet than predefined lists.
Ad.4 Is there any special reason why you want to use .xltx? I used .xls because it is consistent with current 'Import transactions' functionality.
Ad.5 Yes, 2 sheets are enough (3 sheet is unnecessary).
Ad.6 It doesn't matter if these rows are present or not. In 'Import transactions' functionality there are 20 rows without any reference. So I think that we can keep them as they are now.
Ad.7 Where is your attached file?
So, If you want to continue working on file then it would be good if you replace lists with “reference validation lists”.