You want to know what the Exemption Override Code 'Taxable' does.
You have a customer who has transactions that are normally exempt wish to have a taxable transaction (or line, depending on accounting system) without invalidating the customer's Certificate on File.
Avatax Admin Console
You may enter a custom Entity Use Code override called 'Taxable' on both the line and document level, depending on your accounting system. You may need to create this custom entity use code. Refer to other Avalara Help Center articles for how to add entity use codes for availability.
You must ensure this setting is turned on in your Admin Console. Settings > Exemption Certificate Options > Enable Exemption Certificate Management System (ECMS) is checked, and Exemption Override Code: is 'Taxable'.
The Entity Use Code 'Taxable' only makes customers with an exemption profile in the Admin Console ECMS have tax on a single transaction. They must have a certificate on file.
Otherwise, if you aren't using ECMS/Certcapture for exemptions, you'll need to create an Exempt Entity tax rule in Admin Console > Organizaions > Custom Tax Rules. Make a taxability type rule > Taxable: No > Entity Use Code: Type 'Taxable', set your date and the entity use code override 'Taxable' will work for all customers.