The ECC used by buffered modules is the "regular ECC RAM" where the memory bus is widened (and chip count increased) to accommodate carrying the extra ECC information between the DRAM controller on the CPU and the memory modules, with the ECC calculations and corrections done on the CPU and errors surfaced to the firmware and/or OS, giving ECC protection not just for data at rest but also in transit. But the actual ECC bits computed, transferred and stored for each word of DRAM are never directly exposed to software to access.

There is also on-die ECC used by all recent DRAM as a consequence of shrinking memory cell sizes and spacing in the latest DRAM fabrication processes. That on-die ECC is what's unfortunately invisible to the host system, and only really useful for protecting data at rest, not in transit. The existence of on-die ECC has most commonly been publicized in the context of DDR5, but really has nothing to do with what DRAM interface standard is used because the on-die ECC happens entirely on each individual die.

> only really useful for protecting data at rest, not in transit

Which makes it more annoying that normal DDR doesn't also have the ability to add link-ECC like LPDDR.