How to identify downloaded content?

[verypsb]

I collected many CD singles through the years, often multiple CD singles from the same song. I distinguish the versions with a [Disc 1], [Disc 2] in the album title. (Or with discnumbers in the tags) Sometimes that's not enough, and I need to add something like [EU] or [US] in the album title. In the modern age there are several single versions from songs, only available form download sites. How do most people tag these downloads, to distinguish them from old fashioned physical media from the same song? Something like [iTunes], [Juno], [Beatport] or [DWNLD] in the album title?