this is my first posting, so please be kind

I really, really love play's feature of being able to scan RG and write it to the Metadata.
I am using Rockbox on my iPod Nano 1st Gen. and foobar2000 is the only application I miss - when using OS X. play fills that gap almost perfectly. but one thing does not work as expected:
Rockbox does not seem to recognize the RG values in mp4/m4a-files scanned with play. With foobar2000 it does. Is this because foobar uses some obscure tagging scheme, is it play's fault or is it something with rockbox?
I don't know anything about tagging mp4-files. AFAIK mp4 does not use id3 or ape. what does it use? and how can I fix this? what is the difference between play's tagging and foobar's?
thanks in advance and have a nice day!
edit: I am sorry, feel free to move this topic as appropriate.