Does Spotify play the downloaded song while I'm online to save my cellular data?

[GlowingEye]

I'm not sure if Spotify streams a song again although it's already downloaded, just to save my cellular data 'cause it seems to use my internet instead of the downloaded version. So is that right or is it an issue? If not, it would be a good idea for the future... Correct me if I'm wrong or if this isn't the right place to ask that question, I'm new to the community section...