This is something I saw asked about on another forum, and a search of the Autodesk Discussion forum showed it had been submitted to development (supposedly) back in 2006 as an idea. I don't personally have a need for this right now, but I can see it being a very useful tool. The idea is to allow custom symbol promted entries to use a drop down list of choices. Example given was for a material note, with a short list of available colors. Rather than having to type the color in each time, the original poster hoped it would be possible to simply create a list of the options, and have the user select from that list when inserting the symbol in a drawing.