Synced content and data usage

Hello,

I have a question about synced playlists. Imagine that I have a playlist that I have synced for offline playing in my iPhone. I also have access to internet and the app shows by default the connection to my server. Would the app reproduce the local content even if I browse and reach it in the server window? I am concerned about data usage due to an oversight when playing music.

It'd be ncie to confirm that for all synced content, the app does not stream or download any infomration and instead jsut reprouce it locally, regardless of how I access that content.

Thanks in advance!!