Thats how ActiveRecord implements inheritance.
Banking::Token < ActiveRecord::Base is the base class working against payment_tokens table (all records). Which means any select from BankingToken will filter against all the records in payment_tokens table.
Banking::PaymentToken < Banking::Token inherits from Banking::Token which means it works against the same table but filters by default only type = "Banking::PaymentToken",
(type is the default inheritance_column for Active Record)
You wish to create a new class which inherits from Banking::PaymentToken, lets say:
Banking:OtherToken < Banking::PaymentToken
As the mechanism works it will filter by default type in ("Banking:OtherToken").
Now Banking::PaymentToken filter by default will be:
type in ("Banking::PaymentToken", "Banking:OtherToken") because every OtherToken is a PaymentToken as well..