The look-back period is the immediately preceding four sales tax quarters (quarters end 31 May, 31 August, 30 November and the last day of February), reviewed after each quarter ends. A vendor that falls below either test for the four most recent quarters, with no other connection making it a vendor, may file a final return and stop collecting, or may stay registered and keep filing.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.