The two-box Discount, Tax 1, and Tax 2 fields all operate the same way. The little box on the left is for a multiplier to indicate the percentage. For example, if your vendor gives you a 25% discount, enter .25 in the discount multiplier field, and the larger discount field on the right will automatically reflect this. If you are paying sales tax of 6.5%, enter .065 in the tax multiplier field, and the calculation will be made accordingly. The tax fields can be renamed and given default multipliers on the configuration form.
If you don't want the program to figure this for you, but instead wish to manually enter in a value, then leave the little box blank, and enter numbers in the larger boxes. When the little fields are blank (or zero), no automatic calculations will be made.
The Shipping field simply accepts a typed value (no automatic calculations are made on it).
The default settings do not figure tax on shipping costs:
If shipping is taxable in your area, this can be changed in configuration: