In HandiTax, you are able to tag clients by conditions:
Log in to HandiTax.
Go to Clients and click Tag/Untag batches of clients.
Click on Untag All and then click Yes.
Click Conditions then highlight the filter Clients With Reference Codes Starting With A to C and click Select.
All the clients between A to C should now be showing as tagged.
Note: You are able to make a copy of this filter and change the letters in the Expression box to your preference. If you are not sure on how to do this, then read this guide on how to create basic expressions.