Two things that happen with the cart during tax implementation are that new entries are written to the cart and previous entries are deleted from the cart. A tax implementation is an integration that calculates and applies tax rates and amounts to a cart or an order based on various factors, such as product type, price, quantity, location, and tax rules. A tax implementation can use either an external tax service provider or custom Apex code to perform the tax calculation. When a tax implementation is invoked for a cart, it writes new entries to the cart with a type of Charge and a charge type of Tax. These entries represent the tax amounts and adjustments that are applied to the cart. Before writing new entries to the cart, the tax implementation deletes any existing entries with a type of Charge and a charge type of Tax from the cart. This ensures that the tax calculation is accurate and up-to-date based on the current state of the cart and avoids any conflicts or inconsistencies with previous tax entries. Previous entries are not copied to another object or modified with the new tax calculation, as these are not valid actions for handling existing tax entries. Salesforce B2B Commerce Developer Guide: Tax Integration,B2B Commerce Developer Guide: Tax Calculation Flow