A few general rules of thumb:
1. Macs can only boot off the installer disc that shipped with them, and newer retail discs to the limits of their installed hardware. Those limits are clearly stated in the readme that came with the operating system, and the installer box that the operating system shipped with. If you can't find that information, please ask in the forums and someone can point them out.
Note: Macs that shipped new after Mac OS X 10.7 (Lion)'s release did not ship with an operating system disc. However, if they were not yet refreshed after Lion's release, calling AppleCare might result in a system specific disc they can install on with some version of 10.6. The Macs where this was a possibility, are listed at the bottom of https://discussions.apple.com/docs/DOC-2455
For all newer Lion released Macs, the recovery process is outlined in this article.
2. Model specific installer discs are clearly labelled for the specific model in question, though not always for the specific vintage of that model. Those discs can only work on the vintage and model of the disc in question. That means an iMac G5 disc will not work on an iMac Intel or a G4. In event you are missing the disc and bought it from an authorized reseller, call AppleCare at the number closest to you in the link below:
Usually they can send you extras for a nominal fee. In event you bought from a non authorized-reseller, ask the reseller to contact AppleCare for you in case they lost they discs. You'll need these discs for certain hardware test functions, prebundled software not included with the retail operating system that came out the same time as your Mac: including in some situations parts of iLife, Classic and AppleWorks installers, games excluding Chess, Lemkesoft's GraphicConverter, Intuit's Quicken, and Quickbooks, and MacKiev's World Book Encyclopedia. If unsure whether a specific software was bundled with your Mac, ask in the relevant forum after identifying your Mac.
3. When installing via target disc mode, note the sequence stated in this user tip:
4. Booting off 'C' key does not always work, using the Option/Alt key allows you to use the Startup Manager when this does not work:
5. A machine older than 4 years may need a backup (clock) battery replaced for booting from the installer to work. Your clock may still be working, but installing from CD might not be. Here's a listing of Macs that have clock batteries:
6. Some up to date program CDs will boot Macs, but usually only those which both have the previous operating system installed, and were Macs that were released within 3 weeks of the up to date program operating system's release. These discs were labelled Update or DropIn.
In event none of these rules appears to apply to why your disk won't boot or install, feel free to ask in the forums or call AppleCare:
Note: Observe the license restrictions of the installers.
*and were Macs that were released within 3 weeks of the up to date program operating system's release.*
What do you mean by that? I use a Leopard CPU Drop In disc on my PowerBook G4. Definitely not within 3 weeks of Leopard's release. I think your wording is a little confusing.
If you read the Up to date program description, it was only designated for Macs that were released within 3 weeks of the operating system. I.e April 8th through April 25th 2005 Tiger Up To date compatible Macs were released. October 1st through 26th, 2007 Leopard Up to date compatible Macs were released. I don't know what the dates were for the other updates. If you did a Leopard CPU Drop In Disc on a Powerbook G4, you were lucky it worked. There are some exceptions of what work that aren't well publicized. It really is only designated for use of Macs released in that time period that didn't have enough time to have the actual prebundled software include the latest operating system.