The standard tier is defined by us and is used as a Default. A tier being default means that when an affiliate sends traffic to an offer, and their tier is not assigned on the offer the affiliate falls into the payout of a Standard tier. Pricing of an offer is required to have the Standard tier and in all cases, this behavior is needed.
Custom tiers build the structure of tiers. They can be present on top of the Standard tier.
They can be created by going to the settings in the Affiliates section then to the Tiers page.
Newly created tiers are not automatically assigned to existing offers. All offers that are supposed to contain a newly created tier need to be updated to contain the tier. If a tier is not added and traffic of an affiliate is sent to the offer then the affiliate gets a payout of the default tier - the Standard tier.