The latest BlackBerry 10 OS update to 10.3.1 brought many new features and imnprovements. One new feature is the addtion of data monitoring. Before BlackBerry 10.3.1 you had to make use of apps in order to track your data usage, while those apps still exist, you now have a native option. Most will say it now renders those apps obsolete but it really depends on the features and choice is always good.
The native data monitor can be found by going to Settings->Data Management. You will be presented with a graph that shows your data usage. To set your usage cycle tap Go to the Data Usage Settings screen to set up information about your monthly data limit, monthly usage cycle and notification options. Just enter your data allowace in the Mobile Data Limit field, then use the slider to set the Early Data Limit Notification, where it will warn you when you've hit a certain percentage of your data allowance. It will automatically notify when you hit 100 percent data usage. The last option is the Monthly Reset Date, which is the date your allowance resets each month.
As far as accuracy goes, I found it very close to the actual data as shown with my carrier. BlackBerry does include a disclaimer that states 'Data usage calculations are estimates only and not intended to be used for billing purposes'. This can be said for all the third party data monitors apps too, so do take note. It helps to give you an estimate anyway. Of course, if you have plenty of data, you might not need such a function. It's nice that we now have a native option.