This is a feature which helps you identify how local results are being pulled into SERPs for your given key phrases.
This helps you track position of local results within the local pack, as well the overall ranking position on the page. So you should see this 'letter' next to all local results on both Google & Bing.
For instance if you see a number '5' followed by letter 'C'. This means that your website is in 5th position on the page and in 3rd position ('C') in the local pack itself.
We don't currently have this tracking for Yahoo - just Google & Bing.
Also, this only applies to the results in the main search engines - not the pure Maps or Local versions.
For more information check out our blog post about this feature here.