Your audio collection is probably much bigger than the storage on your portable allows for.
An option is not only to transfer part of your collection to the portable but convert it to a lossy format at the same time.


One option is to do it manual.
You build a separate library that fits on the portable and has all the audio in high bitrate  MP3 or AAC
However, if something changes you need to maintain both libraries.


A better strategy is to let the media player do the job.
You make a playlist, a list of songs you want to have on your portable.

On connecting the portable to the media player it will compare what is in the playlist and what is on the portable.
If there are differences it will sync so all what is in the playlist but not on the portable will be transferred to the portable.

Likewise if date modified on the playlist is more recent, the file will be transferred as well.

This will happen if you edit the tags.

It might also happen if you have changed nothing!
Often media player do write the playcount as a tag in the audio file.
This forces an update of the date modified.


It might also delete anything on the portable but not on the playlist.


Often synchronization is combined with transcoding.
Transcoding is converting audio formats on the fly.