Three claims that are often combined
Sample-rate matched means the source and device rates are compatible without a cross-family conversion. No resampling detected means the inspected software path is not changing the rate. Bit-perfect additionally requires the sample values to remain unchanged through decoding, gain, mixing, effects, format conversion, and delivery.
A green rate indicator can support the first two claims. It cannot, by itself, prove the third across hardware and software that the player cannot inspect.
What Nyquist controls on macOS
Before building its PCM graph, Nyquist can select a CoreAudio device, set its nominal rate to the file rate or a clean same-family multiple, wait for the clock to re-lock, and optionally request exclusive ownership. The signal path then compares source and output rates and reports whether resampling is in effect.
Nyquist uses AVAudioEngine, whose mixer operates in
32-bit float. The path is designed to be sample-rate correct and
resample-free when the status says so, but that architecture is not
proof of bit-identical integer PCM from file to DAC. The website
therefore describes those observed conditions instead of
guaranteeing bit-perfect PCM.
DSD over PCM is different
For DSF and DFF on macOS, Nyquist can package the DSD payload in DSD over PCM frames for a compatible USB DAC. The PCM-looking frames are a transport envelope; the DAC recognizes the markers and extracts the DSD data rather than treating it as ordinary PCM. This route depends on compatible hardware.
Original-file network delivery
For UPnP/DLNA playback, Nyquist sends the original file without transcoding. That verifies what leaves Nyquist, not what the receiver does after accepting it. A receiver may decode, resample, apply volume, or reject the format according to its own implementation.
Routes with less control
Bluetooth normally applies a transport codec and should not be described as bit-perfect PCM. AirPlay behavior varies by sender, protocol version, and receiver. iOS, iPadOS, and tvOS own more of the final audio route than macOS does for a directly selected CoreAudio device.
Related: using a USB DAC, Bluetooth vs. USB audio, and Nyquist's signal-path view.