MODE 1 and 2
To achieve the first level of Photo CD compatibility, Kodak's Photo CD requires a CD-ROM drive that provides Mode 2/Form 1 services.
CD-ROM Mode 1 is the standard ability to read CD-ROMs. A CD-ROM drive must support this mode to be considered a CD-ROM drive.
CD-ROM Mode 2 allows for additional capabilities. In relation to Photo CDs the CD-XA Form 1 is the most important element. Mode 2/Form 1 allows the Photo CD to be recognized and read.
CD-I
Philips CD-I CD-ROMs come in two varieties; either it is an interactive multimedia title or it is a MPEG encoded video or feature length movie.
The interactive multimedia titles need the Philips game controllers, and OS9 which would not be compatible with the Macintosh. The MPEG encoded CD-ROMs need special MPEG playback hardware that is not yet available for the Power Macintosh line.