A remote seller over the threshold registers with the Department and collects Alabama tax, in practice through the Simplified Sellers Use Tax (SSUT) program, which collects a single flat-rate tax in place of state and local sales and use taxes; the Department's FAQ explains that collecting and remitting SSUT relieves the seller and the purchaser of any further state or local sales or use tax on those sales. Because the test is the prior calendar year, the obligation runs for the calendar year that follows a year over the threshold.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.