Finally some visuals to go with info I had saved from a convo with a Rivian rep.
"I don't have a diagram. But there are three top tether anchors located on the body of the second row of the R1T and there are two sets of ISOFIX wire located on the body in the bite line between the seat back and cushion."
"In the R1S, there are three top tether anchors located on the back of the seat backs in the second row and two sets of ISOFIX wire located on the seat line. There are also two top tether anchors located on the back of the seat in the third row."
Car safety seats may be installed with either the vehicle's seat belt or its LATCH (lower anchors and tethers for children) system. LATCH is an attachment system for car safety seats. ... Vehicles with the LATCH system have lower anchors located in the back seat, where the seat cushions meet.