A seller over the threshold registers through MassTaxConnect and collects Massachusetts sales or use tax. The official material read does not set a grace period between meeting the threshold and the start of collection, so the prudent reading is that registration and collection are due once the threshold is met; confirm the start date with the state before relying on any delay.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.