I've moved this post to Server Administration , where your question would probably get more replies.
First, have a look at Keep Data Fresh .
This onlinehelp would help you undertand schedule and task more.
Then return to to your question.
Schedule is what Tableau Server Admin (I suppose it's you) setup.
Task is what the users setup during the publish process. Of course this can be modified afterwards.
What's the data source of your user's live connection?
If it's Excel or CSV, please also refer to Updating Underlying Data in a Tableau Server Workbook That Uses a Live Connection to Excel | Tableau Software