Nitro does not modify original images, so it uses sidecars in all cases. However, Adobe never provided a naming strategy for XMP except for proprietary RAWs, so I had to make one up. So, if you are tagging JPEGs, DNGs, TIFFs, etc. the Nitro sidecar is probably not detected by other apps. If you export to a HEIC, then Nitro will write the ratings into the file.
I plan in the future to let people just use “.xmp” for those sidecars, but that will cause problems if you shoot RAW+JPEG because both R and J will want to write to the same XMP file.