Data sharing is a given amount of data to share across lines.
It’s like a big bowl of data that everyone shares from on the plan, and once it is gone there is no more for anyone, even if you haven't had any!
Lets say its a 10GB Share Plan with 4 lines - one line may use 8GB up in the first week of the month, that would only leave 2GB of data for all 4 lines to share for the rest of that month
Think of it as a pie - do all 4 lines need an equal slice or not? Its all good until someone takes a a bigger slice than they should
Thats what you need to consider when choosing between a multi-line plan thats shared or a multi-line plan that has specific allowances for each line
Either way, once the data allowance is used up then service will either be slowed, data access may be prevented or overage charges may start to rack up.