You often need to collect multiple sales taxes together. Set up the individual taxes first in Setup, Taxes. Tax code is a feature you have to turned on from Setup, Feature Setting, Taxes, check the checkbox Enable Taxes code and then save.
Tax code options would then appear under Setup. You would create a new tax code and then choose the taxes that will be part of the taxes. Check one or more taxes.
If you want to record tax exempt reason and use that as a tax code (to be assigned to a customer record as a default for example), choose a reason under Non-taxable reason. Non-taxable reason is another thing you can set up in Setup menu, non-taxable reasons.
Tax code can be defaulted to a customer record. By assigning a tax code to a customer, you are telling the system all the taxes under the tax code needs to be applied to this customer. If you have a customer with multiple locations and each location needs to be charged a different tax, you can activate the setting (under Setup, feature settings, Taxes, Use Ship-to Party tax code. This will also you to create multiple ship to address for a customer and choose a default tax code for each of the location. Ship to addresses are managed in the customer record. Click on the More button on the customer page.
Tax code can also be default at the product level but generally you would assign tax code to each customer.