When communicating with Avalara, Stax Bill generate a unique identifier in the same format as a UUID or GUID.
Stax Bill sends this in the "code" field to Avalara to make sure that the Invoice has a unique identifier visible between the two systems.
If searching for a specific document in Stax Bill, use the Avalara document search page. The search page can be found under Settings, Financial, Taxation, and then Search (if Avalara Direct is enabled). The "Avalara ID" search option on this page corresponds to the previously mentioned unique identifier.
If looking at a more detailed approach, the Avalara ID is also included in the posted invoice summary export. This can be downloaded by navigating to Invoices, Posted Invoices, selecting a set of appropriate filters, clicking Export, selecting "Summary" and then clicking export. This will provide you with a zipped CSV file, and in that CSV file, the column is labeled "Avalara ID" and aligns with this code.