For a Regular Loyalty Program and a Tiered Loyalty Program, you can specify if the points must be accrued or redeemed on the pre-tax or the post-tax amount. For more information on how you can configure this option, see Configuring a Regular Loyalty Program and Consider only Closed Invoices section of Configuring a Tiered Loyalty Program.
In a Nutshell
Pre-Tax Amount
Does not award points on the tax component of the invoice. Meaning, points are awarded only on the payment which is an income to the organization.
It is equivalent to offering a discount to a guest.
This setting is more organization friendly as points are awarded only on the amount spent by the guest towards a service or product and not the total amount.
Post-Tax Amount
Awards points on the complete invoice including tax. Meaning, points are awarded even on the tax component (not an income to the organization).
It is equivalent to direct payment like cash or check.
This setting is more guest friendly as points are awarded on the total amount spent.
Examples
Below are examples to understand how points are accrued in pre-tax scenario:
Points Accrual: Pre-Tax
If a guest avails a service worth $300, the tax is 5%, and if the guest is entitled to receive 20 loyalty point for every 100 dollars spent, let us see how points are accrued in a pre-tax scenario:

The guest spends $315 in total, but points are accrued on the pre-tax amount of $300.
Points Accrual: Post-Tax
If a guest avails a service worth $300, the tax is 5%, and if the guest is entitled to receive 20 loyalty point for every 100 dollars spent, let us see how points are accrued in a post-tax scenario:

The guest accrues points on the total amount spent. In this case points are accrued on $315 payment.