The short answer
A gas appliance connector is rated for the BTU load it can carry at its length; a connector sized for a tank heater may starve a 199,000 BTU tankless unit. Check the heater's manual for the connection size and allowed connector, and the connector's own rating against the unit's input.
In the catalog
Products mentioned
Questions readers ask
What size gas connector for a tankless water heater?
One rated for the unit's full input at its length, matching the connection size in the manual; a gas fitter confirms it.