The Technology Models are:
A “Model One” seller is a “volunteer seller” that has contracted with a Certified Service Provider (CSP) to perform all the seller's sales and use tax functions, other than the seller's obligation to remit tax on its own purchases.
A “Model Two” seller is a seller that has contracted with a provider to use their CAS. A CAS is designed for a business that wants to use the tax calculation software that has been certified by the states while keeping the responsibility for filing returns and remitting the tax.
A “Model Three” seller has a proprietary system that calculates tax due for each jurisdiction that meets the performance standards set by the states. A seller must have a certain level of sales and a proprietary system that calculates tax due for each jurisdiction.
A “Model Four” seller does not use a CSP or a CAS.