The percentage of the tax out of 100%.
Example: 20 for 20% (UK VAT as an example) or 9.975 for 9.975% (CA QST as example).
This number is treated as a IEEE 754 64-bit float, to avoid floating point issues, currently this field is limited to 5 decimal places
(e.g. 9.975 fits, but a value that likely lost precision like 29.924999999999997 won't).