From RBN/C3 Documentation
Practice sections are available only for Rock Band Network 2 songs. They are placed into the EVENTS track as Text Events. There must be at least one practice section in any song in order for it to correctly compile.
Please make sure that your practice sections are text events, NOT lyrics. If you enter them as lyrics, they will compile in Magma but the practice sections will not work after someone purchases the song.
Autogenerated Practice Sections
If no Practice Sections are placed, Magma will autogenerate practice sections by dividing the song equally into 5, 10, or 20 parts, depending upon the length of the song. The practice sections will be named based on the percentage of the song, for example "10-20%", "25-30%", and so on.
Manual Practice Sections
Practice sections can also be placed by hand. They are placed as Text Events in the EVENTS track at the beginning of the section they correspond to (e.g. the "Chorus" section should be placed at the beginning of the song chorus.)
There may be more than one Practice Sections of the same name. However, when entering Practice mode in Rock Band 3 these will not appear as unique sections. Therefore although it's not required, it's preferable to give distinct names to each song section.
When placing sections, consider how someone might want to practice different parts of the song, and divide the song into logical sections. There is a large list of practice sections with descriptive names; only a subset of these are available from the drop-down menu within Reaper.
Reviewing Practice Sections
Within Audition Mode, the current Practice Section name appears within the display along with the elapsed time and M:B:T Peer Reviewers should check to make sure the Practice Sections are named logically and appear at the correct spot in the song.
Venue Autogeneration and Practice Sections
In order for the Lighting Autogeneration to work properly, at least one each of the following sections must be included: [prc_intro], [prc_verse], [prc_chorus], [prc_bridge], [prc_outro]. Autogeneration sections will work for any name with (for example) "chorus" in it, so [prc_chorus_1], [prc_chorus_3], and [prc_chorus] will work for autogeneration purposes.
The Giant List of Practice Sections
Seriously, it's huge. All Practice Sections