Data usage limit is essentially a built-in bill protector that allows you to set a data budget. Once you set the data usage limit, data will be activated in chunks of 0.5GB packs until it reaches the limit set by you.
For instance, if you have set the data usage limit to 2GB, data will be activated in packs of 0.5GB. So 4 data packs will be activated one after the other until it reaches the limit of 2GB, or you choose to deactivate the service. If you use just 3 packs and then deactivate the service, then you only pay for the 3 data packs that you used. The 4th data pack will not be charged.
Data usage limit can be extended anytime before or during the trip. However, once you purchase the service, the limit cannot be reduced.