Payment gateway fees
What your payment processor takes, per gateway, so net profit is net of the fee you actually pay.
Every card payment costs you a percentage plus a fixed amount, and the rate differs by gateway. Profitrics applies these rules to each order based on the payment method the order was placed with.
The defaults are not your rates
Profitrics ships with 2.90% plus 0.30 as the default, which is a common published card rate and almost certainly not yours. International cards, currency conversion and negotiated rates all move it.
The figure to use is the one on your processor's statement, not the one on their pricing page. Take a month's fees, divide by that month's card volume, and you will have a blended rate that is closer to the truth than any headline number.
Per gateway overrides
Each gateway WooCommerce knows about can carry its own percentage and fixed fee. This matters most where one gateway is much cheaper or dearer than the rest: bank transfer at zero, PayPal above card, cash on delivery with its own handling cost.
A gateway with no override uses the default. Setting an override to zero is a real answer and is different from leaving it blank.
Applying a change
Changed rates apply to new orders at once. Existing orders keep the fee that was calculated at the time until you rebuild them from the Orders tab, which is the same mechanism described in Orders.