Sidssp is correct. iTunes will play play 24 bit files and does not play FLAC. you can easily transcode AIFF or ALAC which are supported by iTunes with free third party software like XLD.
One thing iTunes will not due is automatically switch between 16 bit and 24 bit sample rates and it is slightly tedious process to do so. This is one of the nice features of the third party playback like Amarra, Pure Music and Audivrana Plus which work with iTunes is the auto switching of sample rates. On top of that I, along with many others, hear significant audio quality improvements using one of these for playback allowing iTunes to simply act as your music database manager.
One thing iTunes will not due is automatically switch between 16 bit and 24 bit sample rates and it is slightly tedious process to do so. This is one of the nice features of the third party playback like Amarra, Pure Music and Audivrana Plus which work with iTunes is the auto switching of sample rates. On top of that I, along with many others, hear significant audio quality improvements using one of these for playback allowing iTunes to simply act as your music database manager.