Abacus Accounting assigns the global sales tax rate under File > Setup > Company Preferences > General.
If a matter needs to overwrite the global sales tax rate for fees or costs, simply go to the Matter Maintenance screen for that matter and adjust the Rates tab. simultaneously you assign a tax status (taxable or non-taxable) to each Case Code for fee sales tax, and to each Client Expense Code for cost sales tax. If sales tax for fees varies due to the type of work being done, you can even charge sales tax on a time ticket by time ticket basis. Just be sure the option to show tax status on the time ticket screen is turned on in Workstation Options.