Nero normally writes ISO data in the CD sector format 'Mode 1'. However Nero also supports the so-called 'XA format' for ISO data. The XA format is an extension of ISO structures and can only be burned onto the CD in 'Mode 2' (another CD sector format). 'Normal' ISO Mode 1 tracks behave just like Mode2/XA tracks when read under DOS/Windows and using modern CD-ROM drives.
The latest CD-ROM drives can easily read Mode 1 and Mode2/XA-CDs. However there are still some CD-ROM drives around that cannot read the Mode 1 CDs correctly. Owners of these older drives are advised to write data in the Mode2/XA-format.