A flat earning rate across everything you sell assumes everything you sell can afford it. Most catalogues have several things that cannot.
What usually needs excluding
Gift cards. The important one. Points on gift cards means a customer buys a card, earns points, spends the card, and earns again on whatever they buy with it. You have paid twice for one piece of revenue, and someone will notice.
Loss leaders. The item priced at or below cost to get people through the door. Adding a loyalty reward on top turns a deliberate marginal loss into a larger one.
Sale and clearance stock. Often the right call. The discount was the reward.
Wholesale or trade lines, where the margin already reflects volume pricing and a consumer loyalty rate does not belong.
Services and deposits, where the payment is not really a purchase in the sense the programme means.
How it works
Nominate products individually, or whole categories.
Categories are usually the better tool. Excluding the gift cards category means the gift card you add next year is excluded without you remembering to do anything, which is exactly the kind of decision that gets forgotten.
The estimate stays honest
Excluded items do not count towards the earning figure shown in the cart or at checkout. Products excluded entirely show no earning badge rather than showing zero.
This matters more than it might seem. An estimate that promises points and then under-delivers produces a support conversation and a customer who trusts the programme less. Since the entire purpose of showing an estimate is to influence a decision, it has to be a number the customer will actually receive.
Worth doing before launch
Exclusions are much easier to set before a programme goes live than after.
Adding an exclusion later means customers who were earning on something stop earning on it, and some of them will notice and ask. Working out which lines cannot carry a reward is part of setting the rate, not a tidy-up afterwards.
The other direction
The same targeting works positively, as multipliers. The category that cannot afford points and the category you want to push are both handled by pointing rules at products and categories; one turns earning off and the other turns it up.