I think I'd use a key and a label rather than just a code. I think having the code be the label makes it a little difficult to change things going forward when someone inevitably thinks the label isn't clear enough.

I think I'd change this to:

```
code = models.CharField(max_length=20, unique=True)  # ex. "top_20_localized"
desc = models.CharField(max_length=255)  # ex. "Top 20: Percent Localized"
```