|Custom Stuff 1
Information presented below this line is outdated syntax or information used for Custom Stuff 1. It will not work with Custom Stuff 2.
An ID is an attribute that allows Minecraft to identify a block or item unambiguously. Rather than saving a complicated string of characters (such as those used in a name), Minecraft instead remembers ID values and associates them with the block or item being referenced during gameplay. For that reason, it is very important that each block and item have a unique ID not shared by other attributes.
The values for blocks must range from 125 to 4096 (255 in older versions), including either number, while the range for items is much more expansive; going from 512 to 32000 (again, including both).
The number one cause of crashes when creating Custom Stuff is conflicting IDs. A block or item you create cannot share an ID with another block or item. The source doesn't matter. Vanilla Minecraft and every other mod that adds blocks or items, whether traditional or built on Custom Stuff, all draw from the same pool of IDs, and you can't have a conflict with any of them.
If the only thing that's added to Minecraft is what you generate with Custom Stuff (meaning, no other mods that introduce blocks or items), the above values should all be available for use. Be mindful of the fact that when creating custom stuff it's possible to carelessly overlap one id with another that you create without meaning to (particularly if you like to copy-paste the basics from one file to the next); thus making Custom Stuff conflict with itself and crash Minecraft.