This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
A session is a recorded segment that may contain one or more tracks of any
type. The CD recorder doesn't have to write the entire session at once --
you can write a single track, and come back later and write another -- but
the session must be "closed" before a standard audio CD or CD-ROM player
will be able to use it. Additional sessions can be added until the *disc*
is closed or there's no space left.
This provides a simple and fairly reliable way to write some data to
a disc now and still be able to add more later. The trouble with using
multiple sessions is that, every time you write a chunk of data, you incur
a fairly substantial amount of overhead: 23MB after the first session,
and 14MB for every subsequent session. This overhead lead to the
development of "packet writing", which allows drag-and-drop recording,
but works in an entirely different way (see section (6-3)).
Multisession writing was first used on PhotoCD discs, to allow additional
pictures to be appended to existing discs. Today it's most often used
with "linked" multisession discs, and occasionally for CD-Extra discs.
These require a bit more explanation.
When you put a data CD into your CD-ROM drive, the OS finds the last
closed session on the disc and reads the directory from it. (Well,
that's how it's supposed to work. On some older operating systems and
CD-ROM drives, you may get different results.) If the CD was written in
ISO-9660 format -- most store-bought CD-ROMs are -- the directory entries
can point at any file on the CD, no matter which session it was written in.
Most of the popular CD creation programs allow you to "link" one or more
earlier sessions to the session currently being burned. This allows the
files from the previous sessions to appear in the last session without
taking up any additional space on the CD (except for the directory entry).
You can also "remove" or "replace" files, by putting a newer version into
the last session, and by not including a link to the older version.
In contrast, when you put an audio CD into a typical CD player, it only
looks at the first session. For this reason, multisession writes don't
work for audio CDs, but as it happens this limitation can be turned into
an advantage. See section (3-14) for details. This limitation does *not*
mean you have to write an entire audio CD all at once; see section (2-9)
for an overview of track-at-once writing.
(Some audio CD players do seem to be able to recognize all of the tracks on
a multisession audio disc. Most do not. The only way to know for sure is
to try and see. If you are planning to give an audio CD you create to
others, it would be wise to write it in a single session.)
Note that mixing MODE-1 (CD-ROM) and MODE-2 (CD-ROM/XA) sessions on a
single disc isn't allowed. You could create such a thing, but many CD-ROM
drives will have a hard time recognizing it.
See also http://www.roxio.com/en/support/cdr/multisession.html, which goes
into more depth.
On a Macintosh, discs written in HFS or HFS+ format cannot link files back
to earlier sessions. Adding a new session will cause the previous session
Quick recap: if you want to write some data to a CD-ROM now, and some
more later, you write a single data track in multiple sessions (or with
packet writing). If you want to write some audio tracks to a CD now,
and some more later, you write multiple audio tracks in a single session.