This is another thing we see when upgrading systems; folks want to clean up their picklists but are unable to because no one remembers what certain codes mean. I see this a LOT in the Activity Code and Result Code picklists for History. As these fields only allow three characters, you are forced to use an abbreviation of some kind.
However, you can remark what each "code" means right within the Picklist by using Comments.
1. Bring up any picklist (for instance, let's use the Activty Code on a Phone Call).
2. Click "New" to create a new picklist item.
3. Enter in your code as desired (in this case, "ABC"), then after the code, insert a "//" followed by a description.
4. It should look like this: "ABC //ABC Product Line" or something to that effect.
5. Ok your way out.
Now, when you select that picklist item, everything after the "//" is ignored. This is a great way to annotate each picklist item in your system. I can't tell you how many times I've seen confusion with clients when it comes to their own codes.